Optimise email CSS for speed with Premailer

Speed up emails-on-push by removing all descendant selectors from the
CSS they use, which can make the CSS inlining much more efficient in
certain cases.

See: https://gitlab.com/gitlab-org/gitlab-ee/issues/490

See merge request !4511

Bump nokogiri to 1.6.8

Number of important security and bug fixes. See:
https://github.com/sparklemotion/nokogiri/blob/master/CHANGELOG.md#168--2016-06-06

See merge request !4500

Load knapsack in Rakefile only when is bundled

Fixes `production` builds: https://dev.gitlab.org/gitlab/omnibus-gitlab/builds/85407

cc @marin 

See merge request !4514

Notifications dropdown on project page now has descriptions

## What does this MR do?

Adds descriptions to the notification settings dropdown on project pages. Also changed the order of them to make more sense.

## What are the relevant issue numbers?

Part of #12758

## Screenshots (if relevant)

![Screen_Shot_2016-05-31_at_09.35.24](/uploads/b30d82701a13ec15f672139e1225acf5/Screen_Shot_2016-05-31_at_09.35.24.png)

See merge request !4374

Add License Finder information to contribution acceptance criteria.

To make it clear what should be done when the license finder test fails.

cc: @stanhu 

See merge request !4512

Sub navigation

## What does this MR do?
Styles sub nav   
cc @dzaporozhets  @skyruler @jschatz1 

## What are the relevant issue numbers?
#18058 and #17983 

## Screenshots (if relevant)
![Screen_Shot_2016-06-03_at_12.10.26_PM](/uploads/8b948c32699a93ab0a390bda31e0e411/Screen_Shot_2016-06-03_at_12.10.26_PM.png)
![Screen_Shot_2016-06-03_at_12.10.34_PM](/uploads/241115c35d0618f3de739be9a670416d/Screen_Shot_2016-06-03_at_12.10.34_PM.png)

See merge request !4468

Implement compact side nav

## What does this MR do?
Implements new compact nav design   
Note- new navbar icons are in a separate MR (https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/4268)

## What are the relevant issue numbers?
Closes #18013 

## Screenshots (if relevant)
![Screen_Shot_2016-06-02_at_3.20.37_PM](/uploads/d31bd17ae47743f1a20d7429b4396e84/Screen_Shot_2016-06-02_at_3.20.37_PM.png)   
![Screen_Shot_2016-06-02_at_3.20.43_PM](/uploads/d03a080a57707b6a2161127f3904fcd5/Screen_Shot_2016-06-02_at_3.20.43_PM.png)

cc @dzaporozhets 

See merge request !4447

Instrument `RepositoryCheck::SingleRepositoryWorker` manually

See merge request !4446

This worker is called manually by `RepositoryCheck::BatchWorker` meaning it's not tracked automatically by the Sidekiq middleware.

Add new GitLab CI configuration facade class

## What does this MR do?

This MR is a first iteration of major CI configuration refactoring. See #17139 and #15060 for more details.

## What are the relevant issue numbers?

Closes #17139

See merge request !4462

Number of important security and bug fixes. See:
https://github.com/sparklemotion/nokogiri/blob/master/CHANGELOG.md#168--2016-06-06

Ability to prioritize labels

Closes #14189 

See merge request !4009

Fix knapsack for master

It were mistakenly to invoke `rspec` where it should invoke `spinach`.


See merge request !4510

This is part of #12758

Use Knapsack to evenly distribute specs

Try to distribute all tests evenly accross all nodes to reduce the testing time of GitLab CE.

This uses high amount of auto-scaled machines.


See merge request !4240

Added 8.9 install/update guides

See merge request !4507

[ci skip]

Workhorse to serve raw diffs

Fixes (partially) #13999

Dependent on: gitlab-org/gitlab-workhorse!45

See merge request !4130

Rename ci_commit in application code

This is third iteration of renaming `[ci_]commit(s)` in application code to `pipeline(s)`.


See merge request !4464