There are several changes to this module:

1. The use of an explicit stack in Participable#participants
2. Proc behaviour has been changed
3. Batch permissions checking

== Explicit Stack

Participable#participants no longer uses recursion to process "self" and
all child objects, instead it uses an Array and processes objects in
breadth-first order. This allows us to for example create a single
Gitlab::ReferenceExtractor instance and pass this to any Procs. Re-using
a ReferenceExtractor removes the need for running potentially many SQL
queries every time a Proc is called on a new object.

== Proc Behaviour Changed

Previously a Proc in Participable was expected to return an Array of
User instances. This has been changed and instead it's now expected that
a Proc modifies the Gitlab::ReferenceExtractor passed to it. The return
value of the Proc is ignored.

== Permissions Checking

The method Participable#participants uses
Ability.users_that_can_read_project to check if the returned users have
access to the project of "self" _without_ running multiple SQL queries
for every user.

Caching is now always enabled when rendering attributes as there's no
reason to not cache them. The method Mentionable#all_references has also
been modified to take an optional Gitlab::ReferenceExtractor instance to
allow other code to create one and re-use it between multiple calls.

This ensures all these calls have an author set, allowing the use of
"all" mentions where possible.

This splits the Markdown rendering and reference extraction phases into
two distinct code bases. The reference extraction phase no longer relies
on the html-pipeline Gem (and any related code) and allows for
extracting of references from multiple HTML nodes in a single pass. This
means that if you want to extract user references from 200 comments you
no longer need to run 200 times N number of queries, instead only a
handful of queries may be needed.

This class can be used to lazy-evaluate blocks of code the first time
they're called. This can be useful when a method performs a certain
heavy operation (e.g. a SQL query) that you only want to perform
whenever the result is used for the first time.

This method used to call strip! on input strings which will mess with
the strings if they're re-used or frozen.

Fix right sidebar expand/collapse issue

Fixed branch name of the !4117


------------------

Fixes #15643 

- Fixed a typo in `toggleSidebar` method of `Sidebar` class.
- Fixed a minor styling issue to fix the sidebar to the top of the page.
- Added tests for Sidebar.


## Before
![before](/uploads/c4efe9a64e0ae35f423302753caa6dfd/sFBwnVEpQB.gif)

## After
![after](/uploads/23e6b80888dae475a385912c8c40bdbd/rerKiYC4dk.gif)


See merge request !4292

POC: Updated contrib calendar

In an effort to cut down the JS file size - i've removed the heatmap calendar used for the contributing calendar on users profiles. We already have d3 on the page so why not use it instead of using a library which uses it?

![Screen_Shot_2016-04-27_at_11.08.41](/uploads/64c40f0c766f800fd0e33ac8be7f9644/Screen_Shot_2016-04-27_at_11.08.41.png)

cc. @jschatz1 

See merge request !3944

Mobile navigation fade out

Closes #17339     
Closes #17610  

<img src="/uploads/e382dd363bf8beb28339fa6b152be092/Screen_Shot_2016-05-18_at_10.16.59_AM.png" width="400px">


See merge request !4195

Update state_machines-activerecord from 0.3.0 to 0.4.0.

Includes support for Rails 5, working towards #14286.

See merge request !4295

Add fade out to left side of layout nav; remove dropdown nav helper; remove controls button from mobile

Add new HA diagrams [ci skip]

## What does this MR do?

Adds new HA diagrams created by @lukebabb (Thanks @lukebabb!)

See merge request !4289

No changelog, see commits: https://github.com/state-machines/state_machines-activerecord/commits/master

Includes Rails 5 support.

Fix RuboCop

See merge request !4294

New confirmation email

See merge request !3354

[ci skip]

This makes it easier to debug style rules.

Remove `issue_to_atom` helper



See merge request !4265

Improve design of Pipeline view

## What does this MR do?

Improves current design of Pipelines view when there is multiple stages.
This makes the statuses clickable and makes the view more compact.

## Screenshots (if relevant)

![Screen_Shot_2016-05-21_at_01.20.40](/uploads/dd031b7af7005c7a61f3165fefa8b7c9/Screen_Shot_2016-05-21_at_01.20.40.png)

cc @DouweM @markpundsack @rspeicher @marin 

See merge request !4230

Remove `event_to_atom` helper



See merge request !4234

Fix for import URL URI problem when URL contains a space

Fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/17559

Prevents an already encoded URL to be encoded again - preventing %20 => %25%20

See merge request !4180

Fix forks creation when visibility level is restricted

fixes #10725

See merge request !4283

Updated error message when project limit is zero

When the project limit is zero & the user tries to create a project, the
error displayed is now 'Personal project creation is not allowed. Please
contact your administrator with questions'

Closes #17691

See merge request !4280

Fix groups API to list only user's accessible projects

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/17496

See merge request !1966