Add documentation on enabling/disabling GitLab CI

Closes #4289

See merge request !2320

See merge request !2319

[ci skip]

Add Open Graph data for group, project and commit.

See merge request !2211

Get "Merge when build succeeds" to work when commits were pushed to MR
target branch while builds were running

The Merge when build succeeds service only merges when the MR is
mergeable (open, not WIP, no conflicts).

When the target branch is updated, all affected MRs have their merge
status set to `unchecked`, and the conflicts check will only happen
when `check_if_can_be_merged` is called, which happens when the MR page
is viewed.

When someone enables the automatic merge, the target branch is updated,
no-one views the MR page again, and the build succeeds, the mergeability
check will fail and the MR will not in fact be merged.

This MR makes sure `check_if_can_be_merged` is always called when MR
mergeability is checked.

See merge request !2304

Define a limited set of filters for SingleLinePipeline

Removes the following filters from its parent GfmPipeline:

- SyntaxHighlightFilter
- UploadLinkFilter
- TableOfContentsFilter
- LabelReferenceFilter
- TaskListFilter

Closes #1697 

See merge request !2257

Fix mentionable reference extraction caching.

Fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/4130

Reverts https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/2120 and https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/2283

See merge request !2315

InfluxDB over UDP doesn't use authentication, thus there's no need for
these settings.

See merge request !2316

Enable Microsoft Azure OAuth2 support

Added Microsoft Azure as OAuth2 provider (https://github.com/KonaTeam/omniauth-azure-oauth2) including docs and icon.

See merge request !2187

Transaction timings are also already stored in milliseconds, this keeps
things consistent.

Fix project destroy callback

See gitlab-org/gitlab-ee!107.

See merge request !2307

Import GitHub Pull Requests into GitLab

Fixes #2833

See merge request !2168

This reverts commit d337d5e7.

This reverts commit 4b027bc9.

Fix notification spec

Master was broken :(

See merge request !2306

Fix the abuse report detail URL in the HTML email template

Previous URL resulted in a 404.

See merge request !2310

Ignore config/sidekiq.yml

Removes the following filters from its parent GfmPipeline:

- SyntaxHighlightFilter
- UploadLinkFilter
- TableOfContentsFilter
- LabelReferenceFilter
- TaskListFilter

Closes #1697

Validate README format

Take the first previewable README file as project's README, otherwise if
none file is available, or we can't preview any of them, we assume that
project doesn't have a README file.

Fixes #3978

Update version check images to use SVG

See merge request !2244

See gitlab-org/gitlab-ee!107.