Document fun facts about omnibus-gitlab

I hope this helps developers understand some of the differences.

See merge request !1546

Revert upload refactor

See merge request !1548

This reverts commit 7d5f86f6.

This reverts commit ebd39fc0.

Update API branches documentation [ci skip]

Structure oauth docs

Fixes #2010

@valery please let me know if I used the right terminology (GitLab as an OAuth2 client)

See merge request !1545

Fix push access check when not signed in.

Was causing 500 on MR, Blob, Tree pages.

Fixes https://github.com/gitlabhq/gitlabhq/issues/8637 and https://github.com/gitlabhq/gitlabhq/issues/8813.

See merge request !1542

Fix user page performance

See merge request !1543

Escape process text

Performance improvements

* More efficient DB queries
* Cache ApplicationSettings per request
* Dont query for objects you dont need

See merge request !1540

Add "NEWS" to no_highlight_files

"NEWS" is used in gnu style opensource projects for the changelog. So let's add it to `no_highlight_files`.

See merge request !249

Fix Markdown relative links to files with anchors.

Fixes #2016.

cc @marin

See merge request !1541

The Feature request copy paste text was not properly escaped.

Improve user profile page

See merge request !1539