Upgrade premailer-rails from 1.9.0 to 1.9.2

Includes compatibility with Rails 5.

Changelog:
https://github.com/fphilipe/premailer-rails/blob/master/CHANGELOG.md#v192

Working towards #14286.

See merge request !3643

Fix seed_fu failure with inserting milestones into test DB

Milestones were not being saved due to "invalid state" validation errors

See merge request !3641

Ignore toggling the sidebar for resolutions over 1200px width

Closes #14353 
![sidebar1](/uploads/4b8d0eaa1b320c319239cb572932f740/sidebar1.gif)

See merge request !3547

prevent users from being able to both upvote and downvote



See merge request !3495

This happens because tab's `shown` event is triggered first if we enter directly to the diff page therefore Breakpoints class is not initialized yet.

Includes compatibility with Rails 5.

Changelog:
https://github.com/fphilipe/premailer-rails/blob/master/CHANGELOG.md#v19
2

Working towards #14286.

Remove tab stop from "Write", "Preview", "Go full screen" links

Closes #15089

See merge request !3635

Redesign labels page

closes #14227 

## Desktop
![Screen_Shot_2016-04-05_at_7.19.37_PM](/uploads/a52b764565b6e2739fb6932178f8bde5/Screen_Shot_2016-04-05_at_7.19.37_PM.png)

## Mobile
![Screen_Shot_2016-04-04_at_6.40.57_PM](/uploads/9804b074126fcdb6755918e9f4c472a7/Screen_Shot_2016-04-04_at_6.40.57_PM.png)

See merge request !3507

Milestones were not being saved due to "invalid state" validation errors

Everything on one page.



See merge request !3636

Use raw SQL instead of Rails models in 20130403003950 migration

Closes gitlab-org/gitlab-development-kit#109

Closes https://github.com/gitlabhq/gitlabhq/issues/10123

See merge request !3638

Closes gitlab-org/gitlab-development-kit#109

Closes https://github.com/gitlabhq/gitlabhq/issues/10123

Add a `sidekiq` tag to Sentry Raven context

This will let us filter errors that were/were not in the Sidekiq
environment.

Source: http://stackoverflow.com/a/28370539/223897

Closes #15092

See merge request !3634

Closes #15089

This will let us filter errors by the program environment in which they
were encountered.

Source: http://stackoverflow.com/a/28370539/223897

Closes #15092

Enable asset compression in production.

`config.assets.compress` needed to be explicitly enabled.

Follow-up to !3544. Resolves #14344.

See merge request !3632

`config.assets.compress` needed to be explicitly enabled.

Follow-up to !3544. Resolves #14344.

Diff redesign

Fixes #14400 
Fixes #13500 

![diffs_redesign](/uploads/16da9557e497114d886ef5ece2f306b3/diffs_redesign.gif)

See merge request !3476

Update sprockets to 3.6.0

Sprockets 3.5.0 reintroduces GZIP-ing and 3.6.0 includes performance improvements.
Changelog: https://github.com/rails/sprockets/blob/3.x/CHANGELOG.md

Closes #14344.

See merge request !3544

Ensure schema version matches latest migration

From commit 85cc1729

See merge request !3627

From commit 85cc1729

Sprockets 3.5.0 reintroduces GZIP-ing and 3.6.0 includes performance improvements. Changelog: https://github.com/rails/sprockets/blob/3.x/CHANGELOG.md

Resolves #14344.

[ci skip]

Bump rails to 4.2.6

See merge request !3424

Only load emoji spritesheet when necessary

On a normal issue where the only award emoji displayed by default are
the thumbs-up and thumbs-down emoji, this decreases the page load by
670KB or 250KB depending on pixel density.

Resolves #14334.

See merge request !3449

The emoji_icon helper used to display the award emoji in Issue and
Merge Request views. By default the spritesheet is used, but passing
`sprite: false` to the `emoji_icon` helper makes the emoji render as
separate images.

For award emoji displayed by default (e.g. thumbs-up, thumbs-down, and
any that have been awarded to the issue/MR) the independent images are
used. Only when the full emoji menu is opened does the full spritesheet
load.

On a normal issue this change decreases the page load by 670KB or 250KB
depending on pixel density.

Resolves #14334.

Add a missing Oxford comma in some text on the Activity graph.

Resolves #14668. [ci skip]

See merge request !3625

Redis connection pool

Split from https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/3232

Having an easily accessible Redis connection pool allows us to do more
cool stuff with Redis in GitLab (instead of having to go through e.g.
the Rails cache).

See merge request !3521

Instrument Rails cache code

See merge request !3619

Update gitlab-shell to 2.7.2

See merge request !3594

Resolves #14668. [ci skip]

Use meta key to open todo in new tab

Fixes #15078 

See merge request !3623

Remove TODO for not documented stuff

We should not put a `TODO` in a released product.

See merge request !3553

First pass at a Testing styleguide

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/14121

[ci skip]

See merge request !3465