Fixes #21842

Change response for /ci/api/v1/builds/register.json from 404 to 204

## What does this MR do?

To check if there are new builds scheduled for a runner, runner is sending `POST /ci/api/v1/builds/register.json` requests. If there is a build then a `200` response with build data is returned. However if there is no builds scheduled for this runner it receives a `404 Not Found` response. This may end with a lot of `404 Not Found` lines in logs that are an expected behavior (please read #14445 for a reference).

Since `v1.0.1` version GitLab Runner is ready to receive a `204 No Content` response in case of no builds.

This MR adds a support for this status code for each Runner's version that can be determined while handling `POST /ci/api/v1/builds/register.json` request.

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

Specs in `spec/requests/ci/api/builds_spec.rb`

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

## What are the relevant issue numbers?

Fixes #14445

See merge request !6225

Restore SSH Key title auto-population behavior

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/21823

See merge request !6186

Update gitlab shell secret file also when it is empty

Use `File.size?` instead of `File.exist?` so that secret token is
generated when the file is empty.

See merge request !3774

Import GitHub release notes

## Why was this MR needed?
Release notes aren't currently migrated over when importing from GitHub

## What are the relevant issue numbers?
#20780 

See merge request !5981

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/21823

[ci skip]

[ci skip]

Merge branch '21636-deleting-source-project-with-existing-fork-closes-all-related-merge-requests' into 'master'

Deleting source project with existing fork link should closes all
related merge requests, and does not render `invalid` template for merge
requests with deleted source project.
The merge request without source project can be edited but can't be reopened.

Closes #21636 

See merge request !6177

Gitlab::Checks is now instrumented

See merge request !6318

Avoid protected branches checks when verifying access without branch name

See merge request !6317

Removed horizontal overflow from collapsed issuable sidebar

## What does this MR do?

Uses flexbox to remove any horizontal overflow from the collapsed issuable sidebar

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

## Why was this MR needed?

There was a horizontal scrollbar at the bottom of all collapsed issuable sidebars as seen in the screenshot in #21895.

## Screenshots (if relevant)

![2016-09-11_16.28.30](/uploads/8e01565385b1ff0b736630ac3c24a3a1/2016-09-11_16.28.30.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 #21895.

See merge request !6295

Fixed issue merge-request-info missing on mobile

## What does this MR do?

Removed media query declaration relating to `.merge-request-info` and `.issue-info`.

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

## Why was this MR needed?

Mobile UX

## Screenshots (if relevant)

![Screen_Shot_2016-09-07_at_19.10.39](/uploads/6dc39b72459087826071ba9fbd06da17/Screen_Shot_2016-09-07_at_19.10.39.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 #21877

See merge request !6250

So we have a detailed view of what checks perform bad

GitlabShell verify access sending ‘_any’ as the changes made on the git command, in those cases Gitlab::Checks::ChangeAccess won’t receive a branch_name so we don’t need to check for access to the 
protected branches on that repository. So we avoid some git operations in case the are not cached (empty_repo?) and some database lookups to get protected branches.

These request is happening in every push.

Fix an error where we were unable to create a CommitStatus for running state

Due to severe refactoring of Pipeline we introduced regression in how CommitStatus is handled. We received an report that it's impossible to create a CommitStatus with state `running` when there were not previous status. 

The support for Commit Statuses should be simplified. Right now I'm doing minimal change to move forward and fix a bug, but I'll create a new MR that will move all logic that is now part of `lib/api/commit_statuses.rb` to separate service to simplify the implementation.

This error happens due to the fact that we introduced additional status of builds: `created`.

Fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/21345

See merge request !6107

Remove duplication between project builds and admin builds view

Closes #19694 

See merge request !5680

Include region in S3 configuration

- [x] [Documentation created/updated](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/development/doc_styleguide.md)

See https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/5336

See merge request !5896