API: admin users can sudo commands as other users

Change-Id: I966bfd0ccc4b05925384ecab8c6cbe3c6ba3b667

Change-Id: I305266fe9acbbb5136adeeb52e7e4e1d6629a30a

-Specifying a header of SUDO or adding a :sudo with either user id, or username of the user will set the current_user to be that user if your identifying private_token/PRIVATE_TOKEN is an administrator token

* added description
* simplified form
* enabled gfm autocomplete

Improved large commit handling.

Previously, only number of changed files mattered. Now, number of lines to render in the diff are also taken into account.

A hard limit is set, above which diffs are not rendered and users are not allowed to override that. This prevents high server
resource usage with huge commits.

Related to #1745, #2259

In addition, handle large commits for MergeRequests and Compare controllers.

Also fixes a bug where diffs are loaded twice, if user goes directly to merge_requests/:id/diffs URL.

Import Bare Repositories instructions/description are incorrect

Add group membership api

Change-Id: I3b15ae34c0957a0f4026e1886c92a9770e9d170e

Change-Id: I5b174bba02856ede788dcb51ec9b0d598ea7d0df

* show highlights when replace empty line with content
* show inline diff when replace line with spaces with content

Authenticate or create users from OAuth providers

Add 'notes' to path blacklist, fixes #4967