Refactor merge requests documentation

## What does this MR do?

Add more information on merge requests. CE https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/6202

## Moving docs to a new location?

See the guidelines: http://docs.gitlab.com/ce/development/doc_styleguide.html#changing-document-location

- [x] Make sure the old link is not removed and has its contents replaced with a link to the new location.
- [x] Make sure internal links pointing to the document in question are not broken.
- [x] Search and replace any links referring to old docs in GitLab Rails app, specifically under the `app/views/` directory.
- [x] If working on CE, submit an MR to EE with the changes as well.

See merge request !716

https://gitlab.com/gitlab-org/gitlab-ee/commit/0ec82b0a148d626bfe84135d57fd95f3d7d209b0

API: Restore backward-compatibility for POST /projects/:id/members when membership is locked

Closes #971.

/cc @rdavila

See merge request !715

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

Add documentation on template dropdowns

CE https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/4865

See merge request !710

CE upstream

```
Unmerged paths:
  (use "git add <file>..." to mark resolution)

	both modified:   app/views/layouts/nav/_group_settings.html.haml
	both modified:   app/views/projects/edit.html.haml
	both modified:   app/views/shared/issuable/_form.html.haml
```

Related CE changes https://gitlab.com/gitlab-org/gitlab-ce/compare/26bd854aa19e186f529681612164cbf1d5b386fc...fd1741b47970fc52d994367ba38b5d1353d94725

See merge request !709

Ensure an MR never has negative approvals left

Closes #959.

See merge request !708

Fix test for 'pages are deployed'

Fixing this: https://gitlab.com/gitlab-org/gitlab-ee/builds/3686311

The reason is that `Project#ensure_pipeline` switched the order of arguments, but this test is EE-only therefore this change was missed.

See merge request !711

Change the inline code to codeblocks for the new features doc guideline

Change the inline code to codeblocks for the new features doc guideline
for better clarity.

See merge request !6168

[ci skip]

Port policies to EE

Ports https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/5796 to EE, maintaining EE-only differences.

Assuming this all works, I think we should put this block in CE as well just to prevent future conflicts:

```ruby
    # EE-only
    if defined?(License) && License.block_changes?
      cannot! :create_issue
      cannot! :create_merge_request
      cannot! :push_code
      cannot! :push_code_to_protected_branches
    end
```

In addition to cherry-picking the gitlab-org/gitlab-ce@e71cd7a300017cf85e16de3b1c68fdb25c3a4b4d commit, which is the merge of the Policy refactor itself, I had to cherry-pick gitlab-org/gitlab-ce@6686084c6502f10fd7e6b8963ab52526cb6831bf because it added the `Project#has_wiki?` method that specs depended on.

See merge request !702

Update CHANGELOG with 8.11.4 entries.



See merge request !6159

Remove inconsistent font weight for sidebar's labels

## What does this MR do?
Removes the `.bold` css class from the sidebar label default value so that it is no longer bold 😄 

**None** => None

## Are there points in the code the reviewer needs to double check?
None

## Why was this MR needed?
Fixes a UI inconsistency

## What are the relevant issue numbers?
Closes #21141 

## Screenshots (if relevant)
Before:
![Screen_Shot_2016-08-24_at_9.41.11_AM](/uploads/3e416d45531b7322474922c569225c03/Screen_Shot_2016-08-24_at_9.41.11_AM.png)

After:
![Screen_Shot_2016-08-24_at_9.40.57_AM](/uploads/a4d60d4cd0a57142ad6c0c8a25be5342/Screen_Shot_2016-08-24_at_9.40.57_AM.png)

## Does this MR meet the acceptance criteria?

- [x] [CHANGELOG](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CHANGELOG) entry added
- Tests
  - [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 !5984

Fix inconsistent checkbox alignment

## What does this MR do?
Vertically aligns the checkboxes to the top so that it is aligned with the issue titles

## Are there points in the code the reviewer needs to double check?
None

## Why was this MR needed?
Fixes existing UI inconsistency

## What are the relevant issue numbers?
Closes #21398

## Screenshots (if relevant)
Before:
![Screen_Shot_2016-08-25_at_12.43.21_PM](/uploads/c494d6200e2068feb324db932851cdc0/Screen_Shot_2016-08-25_at_12.43.21_PM.png)

After:
![Screen_Shot_2016-08-25_at_12.41.51_PM](/uploads/42241a150c886592ff92a5840090c589/Screen_Shot_2016-08-25_at_12.41.51_PM.png)

## Does this MR meet the acceptance criteria?

- [x] [CHANGELOG](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CHANGELOG) entry added
- Tests
  - [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 !6032