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

Use more accurate timestamps for InfluxDB.

See merge request !3617

This allows us to track how much time of a transaction is spent in
dealing with cached data.

Use gitlab-workhorse 0.7.2

See merge request !3570

This changes the timestamp of metrics to be more accurate/unique by
using Time#to_f combined with a small random jitter value. This
combination hopefully reduces the amount of collisions, though there's
no way to fully prevent any from occurring.

Fixes gitlab-com/operations#175

API: Expose subscribed? on issues and merge requests

This is the first start for the new `subscribable` API. It exposes the `subscribed` field for issues and merge requests.

Related to #6024

See merge request !3611

Fix Incorrect Quote In Docker Executor Example

## What does this MR do?
The corrected line opened with a regular quote (') but closed with a back-tick (`)  
I believe this is invalid bash syntax

## Are there points in the code the reviewer needs to double check?
Probably not

## Why was this MR needed?
Fixes error when example is copied and pasted

## What are the relevant issue numbers?
None

## Screenshots (if relevant)
None

See merge request !3604

Remove changelog entry for new navigation sidebar.



See merge request !3608

Fix side-by-side code format & commit message wrap

![Screen_Shot_2016-04-07_at_1.31.28_PM](/uploads/bad00284e4dfbec1fdd75220c34f4a98/Screen_Shot_2016-04-07_at_1.31.28_PM.png)

![Screen_Shot_2016-04-07_at_1.32.23_PM](/uploads/7cd344765025e93d0035934a473b4bb3/Screen_Shot_2016-04-07_at_1.32.23_PM.png)

See merge request !3605

Revert "Merge branch 'new-navigation-prototype' into 'master'"

This reverts merge request !3494

See merge request !3607

[ci skip]

This reverts merge request !3494

Revert "Merge branch 'fix-sidebar-exapnd' into 'master'"

This reverts merge request !3520

See merge request !3606

This reverts merge request !3520

Preserve white space



See merge request !3602

Update number of Todos in the sidebar when it's marked as "Done"

Closes #15002 

See merge request !3600