Added System Hooks for push and tag_push

Implements `push` and `tag_push` events for System Hooks.

They are based on Webhooks Events but without any deprecated item. We don't send commits too.
We are implementing this, to be used by Geo repository syncing (gitlab-org/gitlab-ee#76).

These hook events will be sent only when the respective configuration flags are set to true.
UI changes to admin screen will be made in another MR.

See merge request !3744

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

Code is based on Project Webhooks, removing deprecations and without
commits listing.

Improve 'auto fsck' feature

- skip wiki repositories when they are disabled (avoids false alarms)
- more context in admin alert emails

Fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/15332

See merge request !3781

Import milestones from GitHub

Closes #13421

See merge request !3797

Import labels from GitHub

Closes #15292

See merge request !3776

Use GitHub Issue/PR number as iid to keep references

Closes #15294 

See merge request !3759

Fixed issue with multi select not working with arrow keys



See merge request !3792

Add null check to formData.append

Fixes https://gitlab.com/gitlab-com/support-forum/issues/645

See merge request !3799

Change the link color of the target branch in a merge rquest

closes #15296

See merge request !3786

Use raw SQL commands for 20140502125220 MigrateRepoSize

Partial fix for #15210

See merge request !3789

Fixed overlap in header on mobile

![Screen_Shot_2016-04-18_at_08.48.29](/uploads/8f371d0c717a57ca7afb29415e13d96c/Screen_Shot_2016-04-18_at_08.48.29.png)

See merge request !3777

Removes dropzone focus class after dropping file



See merge request !3756

Updated note form focus to better match form-control focus

![Screen_Shot_2016-04-15_at_09.47.52](/uploads/39e02f79a72798ecc6dc946c7b3c8e2e/Screen_Shot_2016-04-15_at_09.47.52.png)

See merge request !3739

Wrap code in parallel view

Code scrolling was breaking the parallel layout. It should wrap on 'Side-by-side' mode

<img src="/uploads/db657e6c7f08ce57f1be615d7193136a/Screen_Shot_2016-04-14_at_4.28.31_PM.png" width="700px">

See merge request !3731

Fixed issue with commit time not using timeago



See merge request !3723

Partial fix for #15210

Fixed issue with spinner not hiding on diff changes

See merge request !3793

Add "action" tag to /internal/allowed API

See merge request !3791

This allows us to re-use any other analytics that rely on the "action"
tag having a value set.

Restrict public users for private instances

Implements #3508

See merge request !3440

With these changes we don’t lost the issue/pr references when importing
them to GitLab.

Count the number of SQL queries per transaction

cc @pcarranza 

See merge request !3783