Add Frontend Development Guidelines to the Developer Documentation

## What does this MR do?

WIP incomplete draft of some Frontend Development Guidelines for contributors to the project, new hires, and for other projects and persons to look to for guidance.

These are all subject to removal, modification, etc. If you disagree with any of these, please feel free to voice your concerns :)

## Why was this MR needed?

There are a number of guidelines we currently follow implicitly, this makes them more official and available for reference/discussion.

It also exists as a reference for certain patterns we use which may be unfamiliar to new contributors, e.g. the per-page JS.

Feel free to give feedback on wording, missing sections/guidelines, whether a guideline should even exist, etc.

See merge request !5477

Updated diff toggle targets and added icon

## What does this MR do?

Adds the new diff toggle icon and alters the toggle targets.

User can now click on the file header bar _(where no other elements are above it, apart from the icon)_ and the `Click to expand` link to expand the diff and no where else.

## Are there points in the code the reviewer needs to double check?

## Why was this MR needed?

## Screenshots (if relevant)

![Screen_Shot_2016-09-02_at_15.35.15](/uploads/c1cb8c0547328153250294d6c95dd96a/Screen_Shot_2016-09-02_at_15.35.15.png)

####  Gif

![2016-09-02_15.34.31](/uploads/abaefaeba9ce8ef129522dae34574c57/2016-09-02_15.34.31.gif)

## Does this MR meet the acceptance criteria?

- [ ] [CHANGELOG](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CHANGELOG) entry added
- [ ] [Documentation created/updated](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/development/doc_styleguide.md)
- [ ] API support added
- Tests
  - [ ] Added for this feature/bug
  - [ ] All builds are passing
- [ ] Conform by the [merge request performance guides](http://docs.gitlab.com/ce/development/merge_request_performance_guidelines.html)
- [ ] Conform by the [style guides](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CONTRIBUTING.md#style-guides)
- [ ] Branch has no merge conflicts with `master` (if you do - rebase it please)
- [ ] [Squashed related commits together](https://git-scm.com/book/en/Git-Tools-Rewriting-History#Squashing-Commits)

## What are the relevant issue numbers?

Closes #21369 

Closes #20326

See merge request !6183

Added soft wrap option to editor (attempt 2 after killing master)

Previously !6188.

## What does this MR do?

Adds a `Soft wrap` button to the editor, when clicked, it wraps the text in the editor and changes to `No wrap`, then when clicked, it unwraps the text in the editor.

## Are there points in the code the reviewer needs to double check?

Let's make sure we dont blow up `master` this time. 😨 😆 

## Why was this MR needed?

## Screenshots (if relevant)

https://youtu.be/8LW5nQsraSM

#### No wrap

![Screen_Shot_2016-09-02_at_19.54.54](/uploads/97f2d1b2d415d03fe1b0be0640ab12e0/Screen_Shot_2016-09-02_at_19.54.54.png)

#### Soft wrap

![Screen_Shot_2016-09-02_at_19.54.45](/uploads/5af425587ce7198e015cce58440971b9/Screen_Shot_2016-09-02_at_19.54.45.png)

## Does this MR meet the acceptance criteria?

- [ ] [CHANGELOG](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CHANGELOG) entry added
- [ ] [Documentation created/updated](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/development/doc_styleguide.md)
- [ ] API support added
- Tests
  - [ ] Added for this feature/bug
  - [ ] All builds are passing
- [ ] Conform by the [merge request performance guides](http://docs.gitlab.com/ce/development/merge_request_performance_guidelines.html)
- [ ] Conform by the [style guides](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CONTRIBUTING.md#style-guides)
- [ ] Branch has no merge conflicts with `master` (if you do - rebase it please)
- [ ] [Squashed related commits together](https://git-scm.com/book/en/Git-Tools-Rewriting-History#Squashing-Commits)

## What are the relevant issue numbers?


Closes #18297

See merge request !6594

Updated tests

Added tests

Added awesomeeeeee icons

Fix lint-doc error

## What does this MR do?
Removes duplicate changelog versions to fix lint-doc error (https://gitlab.com/gitlab-org/gitlab-ce/builds/4621603)

See merge request !6623

Add missing values to linter (`only`, `except`) and add new one `Environment`

Closes #21744 

See merge request !6276

Before rendering `show` template we close open merge request without source project. This way there is no need to render `invalid` template. I think that it's better solution than !6383

See merge request !6478

Update "Installation from source" guide for 8.13.0

Following the steps from https://gitlab.com/gitlab-org/release-tools/blob/master/doc/release-candidates.md#creating-rc1.

See merge request !6565

Added "View wiki pages" to list of actions granted to various user permission levels in a project.

This MR clarifies the actions allowed by various user permission levels in a project to include "View wiki pages"

See merge request !5749

Signed-off-by: Rémy Coutable <remy@rymai.me>

Signed-off-by: Rémy Coutable <remy@rymai.me>

Fixed Session Cookie header

## What does this MR do?

This MR fixes the wiki text header for the Session Cookie section

## Are there points in the code the reviewer needs to double check?

It is markdown so probably not. Two very simple changes - first is replacing a dot with a space (was causing invalid markdown, not rendered as a header) and capitalized the word Cookie to match the other header formatting.

## Why was this MR needed?

Fix the wiki documentation.

## Screenshots (if relevant)

None.

## Does this MR meet the acceptance criteria?

I can't imagine it wouldn't.

- [x] [CHANGELOG](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CHANGELOG) entry added
- [x] [Documentation created/updated](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/development/doc_styleguide.md)
- [x] API support added
- Tests
  - [x] Added for this feature/bug
  - [x] All builds are passing
- [x] Conform by the [merge request performance guides](http://docs.gitlab.com/ce/development/merge_request_performance_guidelines.html)
- [x] Conform by the [style guides](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CONTRIBUTING.md#style-guides)
- [x] Branch has no merge conflicts with `master` (if you do - rebase it please)
- [x] [Squashed related commits together](https://git-scm.com/book/en/Git-Tools-Rewriting-History#Squashing-Commits)

## What are the relevant issue numbers?

I didn't think this warranted an issue as it  is just some simple wiki markdown formatting with little to no consequence.

See merge request !6584

Smaller formatting fix

See the contents, it's just a smaller fix in formatting. No detailed description needed - just keeping the docs clean

See merge request !6614

Fix build sidebar build details padding

## What does this MR do?

Removes a `.block-first` overriding declaration that was added to fix the coverage padding and moved the padding that fixes the coverage block to a `.block.coverage` declaration.

## Are there points in the code the reviewer needs to double check?

## Why was this MR needed?

The build sidebar build details had too much padding as seen in #22529.

## Screenshots (if relevant)

![Screen_Shot_2016-09-24_at_19.38.40](/uploads/2c11a71f3022909fe8a5d1e983445667/Screen_Shot_2016-09-24_at_19.38.40.png)

![Screen_Shot_2016-09-24_at_19.39.06](/uploads/482af29f4bb7e284469466da59901087/Screen_Shot_2016-09-24_at_19.39.06.png)


## Does this MR meet the acceptance criteria?

- [ ] [CHANGELOG](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CHANGELOG) entry added
- [ ] [Documentation created/updated](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/development/doc_styleguide.md)
- [ ] API support added
- Tests
  - [ ] Added for this feature/bug
  - [ ] All builds are passing
- [ ] Conform by the [merge request performance guides](http://docs.gitlab.com/ce/development/merge_request_performance_guidelines.html)
- [ ] Conform by the [style guides](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CONTRIBUTING.md#style-guides)
- [ ] Branch has no merge conflicts with `master` (if you do - rebase it please)
- [ ] [Squashed related commits together](https://git-scm.com/book/en/Git-Tools-Rewriting-History#Squashing-Commits)

## What are the relevant issue numbers?


Closes #22529

See merge request !6506

Fix a confusion in OAuth2 documentation

## What does this MR do?

fixes confusion :)

## Why was this MR needed?

Because no body wants to be confused :)

See merge request !6563

# Conflicts:
#   doc/api/oauth2.md

Initialize Redis pool in single-threaded context

See merge request !6613

Update templates for 8.13



See merge request !6612

Remove the "soon to be deprecated" `alias_method_chain` in favor of `Module#prepend`.

## Are there points in the code the reviewer needs to double check?

Double check whether the behavior of `attr_encrypted_no_db_connection` is still the desired one.

## Why was this MR needed?

The `alias_method_chain` becomes deprecated in Rails 5 in favor of the `Module#prepend` introduced in Ruby 2.0.

This MR prevents future deprecated warnings in light of a possible Rails version bump.

Closes #22302

See merge request !6570

Take filters in account in issuable counters

## What does this MR do?

This merge request ensure we display issuable counters that take in account all the selected filters, solving #15356.

## Are there points in the code the reviewer needs to double check?

There was an issue (#22414) in the original implementation (!4960) when more than one label was selected because calling `#count` when the ActiveRecordRelation contains a `.group` returns an OrderedHash. This merge request relies on [how Kaminari handle this case](https://github.com/amatsuda/kaminari/blob/master/lib/kaminari/models/active_record_relation_methods.rb#L24-L30).

A few things to note:
- The `COUNT` query issued by Kaminari for the pagination is now cached because it's already run by `ApplicationHelper#state_filters_text_for`, so in the end we issue one less SQL query than before;
- In the case when more than one label are selected, the `COUNT` queries return an OrderedHash in the form `{ ISSUABLE_ID => COUNT_OF_SELECTED_FILTERS }` on which `#count` is called: this drawback is already in place (for instance when loading https://gitlab.com/gitlab-org/gitlab-ce/issues?scope=all&state=all&utf8=%E2%9C%93&label_name%5B%5D=bug&label_name%5B%5D=regression) since that's how Kaminari solves this, **the difference is that now we do that two more times for the two states that are not currently selected**. I will let the ~Performance team decide if that's something acceptable or not, otherwise we will have to find another solution...
- The queries that count the # of issuable are a bit more complex than before, from:

    ```
   (0.6ms)  SELECT COUNT(*) FROM "issues" WHERE "issues"."deleted_at" IS NULL AND "issues"."project_id" = $1 AND ("issues"."state" IN ('opened','reopened'))  [["project_id", 2]]
   (0.2ms)  SELECT COUNT(*) FROM "issues" WHERE "issues"."deleted_at" IS NULL AND "issues"."project_id" = $1 AND ("issues"."state" IN ('closed'))  [["project_id", 2]]
   (0.2ms)  SELECT COUNT(*) FROM "issues" WHERE "issues"."deleted_at" IS NULL AND "issues"."project_id" = $1  [["project_id", 2]]
    ```

    to

    ```
   (0.7ms)  SELECT COUNT(*) AS count_all, "issues"."id" AS issues_id FROM "issues" INNER JOIN "label_links" ON "label_links"."target_id" = "issues"."id" AND "label_links"."target_type" = $1 INNER JOIN "labels" ON "labels"."id" = "label_links"."label_id" WHERE "issues"."deleted_at" IS NULL AND ("issues"."state" IN ('opened','reopened')) AND "issues"."project_id" = 2 AND "labels"."title" IN ('bug', 'discussion') AND "labels"."project_id" = 2 GROUP BY "issues"."id" HAVING COUNT(DISTINCT labels.title) = 2  [["target_type", "Issue"]]
   (0.5ms)  SELECT COUNT(*) AS count_all, "issues"."id" AS issues_id FROM "issues" INNER JOIN "label_links" ON "label_links"."target_id" = "issues"."id" AND "label_links"."target_type" = $1 INNER JOIN "labels" ON "labels"."id" = "label_links"."label_id" WHERE "issues"."deleted_at" IS NULL AND ("issues"."state" IN ('closed')) AND "issues"."project_id" = 2 AND "labels"."title" IN ('bug', 'discussion') AND "labels"."project_id" = 2 GROUP BY "issues"."id" HAVING COUNT(DISTINCT labels.title) = 2  [["target_type", "Issue"]]
   (0.5ms)  SELECT COUNT(*) AS count_all, "issues"."id" AS issues_id FROM "issues" INNER JOIN "label_links" ON "label_links"."target_id" = "issues"."id" AND "label_links"."target_type" = $1 INNER JOIN "labels" ON "labels"."id" = "label_links"."label_id" WHERE "issues"."deleted_at" IS NULL AND "issues"."project_id" = 2 AND "labels"."title" IN ('bug', 'discussion') AND "labels"."project_id" = 2 GROUP BY "issues"."id" HAVING COUNT(DISTINCT labels.title) = 2  [["target_type", "Issue"]]
    ```
- We could cache the counters for a few minutes? The key could be `PROJECT_ID-ISSUABLE_TYPE-PARAMS`.

A few possible arguments in favor of "it's an acceptable solution":
- most of the time people filter with a single label => no performance problem here
- when filtering with more than one label, usually the result set is reduced, limiting the performance issues 

## What are the relevant issue numbers?

Closes #15356

See merge request !6496

Expose the Koding application settings in the API

## Why was this MR needed?

When saving the GitLab application secrets in Koding, and authorising your admin user to have access to the UI, we want to let Koding enable the integration, and populate the url in GitLab for the user.

## What are the relevant issue numbers?

Fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/22705

See merge request !6555

This side-steps the need for mutexes and whatnot.

Signed-off-by: Rémy Coutable <remy@rymai.me>