Add migration-related tips to the "Merge Request Guidelines" doc

## Why was this MR needed?

To avoid issues like #20606 cropping up again.

See merge request !5700

Document that webhook secret token is sent in X-Gitlab-Token HTTP header

## What does this MR do?

Note that the secret token is sent in the X-Gitlab-Token header on the webhook documentation page, as well as directly below the secret token field on the webhook settings form.

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

No.

## Why was this MR needed?

It took me a while to figure out how to verify the token in my hook endpoint. Issue #18256 is where I found how to do it.

## What are the relevant issue numbers?

#18256 

## Screenshots (if relevant)

## Does this MR meet the acceptance criteria?

- [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)
- [ ] API support added
- Tests
  - [ ] Added for this feature/bug
  - [x] All builds are passing
- [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)
Closes #18256

See merge request !5664

fix: respect data-attribute 'skip-users' in user_selects

Backport https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/604 to CE, so that JS file is the same in both versions. Thanks @hwdegroot!

No CHANGELOG needed because this code path isn't used anywhere in CE at the moment.

See merge request !5685

Simplify feature introduction note



See merge request !5705

[ci skip]

[ci-skip]

Add doc guidelines on documents naming and location

## What does this MR do?

Add guidelines on the structure of the documentation.

## What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ce/issues/3349

See merge request !5641

Add 'run tests' development docs from GDK

This adds some documentation that lived in
https://gitlab.com/gitlab-org/gitlab-development-kit but which belongs
more in here.


See merge request !5684

Add examples to repository files API

## What does this MR do?

Add examples to repository files API.

## Why was this MR needed?

There were no examples.

## What are the relevant issue numbers?

fixes #19779


See merge request !5465

Added tests to dateTime utility getDayName method

## What does this MR do?

It was missing tests so i've created some tests to check the correct day name is returned

See merge request !5324

Hope to avoid CHANGELOG conflicts. Credit myself for the MR.

Added changelog item for issuable form dropdowns

## What does this MR do?

Adds CHANGELOG item about the new dropdowns in the issuable form

See merge request !5266

Fixed enter submitting form in dropdown

## What does this MR do?

When searching for a branch in the ref switcher & then pressing enter it would submit the form without anything. This instead doesn't submit the form without anything being selected in the dropdown.

Also improved the UX by making it submit with `Turoblinks` as currently no loading bar is shown so the user has no idea if anything is happening.

## What are the relevant issue numbers?

Closes #19549 

See merge request !5111

Update timeago to shorter representation

## What does this MR do?
Shortens representation of `Finished at` block in pipelines & builds

## What are the relevant issue numbers?
Closes #19752    
Part of https://gitlab.com/gitlab-org/gitlab-ce/issues/18920

## Screenshots (if relevant)
![Screen_Shot_2016-07-14_at_8.54.44_AM](/uploads/85000549c49165d4481422eac5eb0ca9/Screen_Shot_2016-07-14_at_8.54.44_AM.png)

See merge request !5225

* Respect parameters of each individual select,
  instead of the global.
* Update skipped users in approver search when removed from default approvers.

Bump gitlab_git to lazy load compare commits

See merge request !5667

Guide about what requires downtime

## What does this MR do?

This MR adds a guide describing various SQL operations and whether they need downtime or not.

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

Spalling and styling of the text mostly.

## Why was this MR needed?

Developers aren't always aware of the impact of certain operations and the documentation of MySQL and PostgreSQL can be quite confusing at times.

## Screenshots (if relevant)

![screenshot](/uploads/d8afd4bd3755d26e4786dfafecfa9368/screenshot.png)

<img src="https://emoji.slack-edge.com/T02592416/trollface/8c0ac4ae98.png" width="60" />

## Does this MR meet the acceptance criteria?

- [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)
- Tests
  - [ ] All builds are passing
- [ ] 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)
- [x] [Squashed related commits together](https://git-scm.com/book/en/Git-Tools-Rewriting-History#Squashing-Commits)

See merge request !5672

Ignore URLs starting with // in Markdown links

## What does this MR do?

Render `[foo](//bar/baz)` as `<a href="//bar/baz">foo</a>`.

## Why was this MR needed?

`[foo](//bar/baz)` currently renders as `<a href="//bar/gitlab-org/gitlab-ce/master/baz">foo</a>`

## What are the relevant issue numbers?

fixes #7032

See merge request !5677

Add issue description to txt/plain emails

## What does this MR do?
Adds issue description to new_issue mail when in text/plain content-type

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

## Why was this MR needed?
Because clients that are rendering emails in text mode can see issue description 

## What are the relevant issue numbers?
#1821 

## Screenshots (if relevant)

## Does this MR meet the acceptance criteria?

- [x] [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
  - [x] All builds are passing
- [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)

See merge request !5663

Add unfold links for Side-by-Side view

## What does this MR do?

Add unfold links for Side-by-Side view and refactor some diff related code

## What are the relevant issue numbers?

Closes #3877 

## Does this MR meet the acceptance criteria?

- [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 [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)

See merge request !5415

Upgrade newrelic_rpm from 3.14.1.311 to 3.16.0.318.

## What does this MR do?

While we intend to remove newrelic in the future, it'll probably not happen until 9.0 due to. In the meantime, I'd like to update the gem so it won't block Rails 5.

Changelog: https://github.com/newrelic/rpm/blob/master/CHANGELOG

## Why was this MR needed?

Rails 5 compatibility.

Relevant issues: #14286, #12860 

See merge request !5553

Move abilities by subject class to a dedicated method

## What does this MR do?

Twin of gitlab-org/gitlab-ee!622.

It moves the `case` that returns abilities depending on the subject class to a dedicated method.

This should avoid lame conflicts when CE to EE since the EE version assign the result of the `case` to a variable.

## What are the relevant issue numbers?

Closes #20621.

See merge request !5668

Add a data migration to fix some missing timestamps in the members table (again)

## What does this MR do?

Repeats an earlier migration to fix historic bad data in the members table (missing created_at and updated_at fields)

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

I'm expecting the WHERE clauses to be fast enough, and to return few enough rows, that the migration doesn't need to use batches, but I'm not too familiar with the size of these tables in the wild, so perhaps that's a poor assumption. 

## Why was this MR needed?

8.10 introduced a dependency on the  `members.created_at` field in the project and namespace member view. If bad data is present, viewing the list of members now results in an NoMethodError and a 500 response from GitLab. Although the previous migration should have fixed all bad rows, we have evidence that it didn't in at least one case, despite the migration claiming to have run in the past.

## What are the relevant issue numbers?

#20568 

## Screenshots (if relevant)

## 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 [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)

Closes #20568

See merge request !5670

Set consistency in list text height css

Fix line height of list row. Now its 42px everywhere. Before this merge request certain row height varied from 40 to 45px. Sometimes even inside one list. 

cc @jschatz1 @connorshea 

See merge request !5666

Migrate protected branch access levels to match constants in `Gitlab::Access`

Closes #20606 

Context: https://gitlab.com/gitlab-org/gitlab-ce/issues/20606#note_13561628

See merge request !5658

Fix `#down` for two protected branches-related migrations.

- The migrations called `add_column_with_default` with a `null` option, which the Rails `add_column` method accepts. This fails because `add_column_with_default` expects an `allow_null` option instead.

- The migrations have been fixed to use `allow_null`.

See merge request !5660

This guide describes various commonly used operations and the impact
they have on the availability of a GitLab instance.

Refactor build artifacts documentation

## What does this MR do?

- Split user and admin documentation
- Use new location paths
- Add new Continuous Integration guide for the Admin area
- Link to new guide from the Admin area

## What are the relevant issue numbers?

Fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/20612

See merge request !5662

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