Add avatar to issue and MR pages header

closes #14395
# For Issues

### Before
![Screen_Shot_2016-03-18_at_4.27.25_PM](/uploads/b097c8e304c9737e2f46ee25667829bf/Screen_Shot_2016-03-18_at_4.27.25_PM.png)

### After
![Screen_Shot_2016-03-18_at_4.22.10_PM](/uploads/f7d2b12c9455950c2e660b33e27c1827/Screen_Shot_2016-03-18_at_4.22.10_PM.png)

# For Merge Request

### Before
![Screen_Shot_2016-03-18_at_4.27.09_PM](/uploads/50c7b6ee346f749f448647004ae6031b/Screen_Shot_2016-03-18_at_4.27.09_PM.png)

### After
![Screen_Shot_2016-03-18_at_4.24.17_PM](/uploads/abd37ccf030e0b6d10521a6937206de6/Screen_Shot_2016-03-18_at_4.24.17_PM.png)

See merge request !3301

Refactor variables

Fixes #13803 and other color inconsistency across application

cc @jschatz1 @skyruler 

See merge request !3304

Trigger a todo for mentions on commits page

Closes #14006 

* Screenshot:

![todo-commit](/uploads/5d34de0b7afcea7548123dafddf60c45/todo-commit.png)

See merge request !3262

Fixes issue with filter label missing on labels & milestones

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/14407

See merge request !3297

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

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

Prevent a 500 when a repository's root_ref is nil

Closes #14259

See merge request !3278

Since we're not actually returning a todo reference.

Dashboard labels

Previously because there were no JSON endpoint for labels or milestones
it was fetching HTML and parsing that. This is wrong.

It now fetches from a JSON endpoint.

This also fixes the dashboard/issues page not loading labels or
milestones as the path used to be only for a single project. So instead
I have created a endpoint for labels & milestones JSON on dashboard.

See merge request !3253

Fixed other issues based on feedback

They would try to load JSON from a project even though it isn't a single project

Fixes issue with close button not working on MR

Closes #14383

See merge request !3295

Improved issue sidebar

Updated the spacing throughout the sidebar so that it fits better on smaller screens. Also adds a more participants button to show any other participants after the first row.

Closes #13353

See merge request !3097

Add note in upgrade guide to choose the correct version tag

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/3448

@rspeicher I'm assigning to you since as a release manager you should know better. Let's keep it in this guide for now and then start copying it in the future ones.

See merge request !2284

[ci skip]

Fix MergeRequest#source_sha when there is no diff

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/14170

### Overview

This MR fixes an unhandled Exception when visiting the page of an open Merge Request  without diff.

### Description

`MergeRequest#source_sha` is expected to return the sha of the source branch last commit. But when an open Merge Request has no diff (e.g. all commits have already been merged to the target branch), `merge_request.source_sha` incorrectly returns `nil`.

This was without consequences before – but since !2217 was merged (a few days ago), it makes `Gitlab::Git::Commit.between` raise an "Unexpected nil argument" exception. This can be reproduced when visiting the http://localhost:3000/gitlab-org/gitlab-test/merge_requests/2 page on a fresh local Gitlab setup.

This MR fixes the crash, by making sure that `source_sha` returns a
correct result even when there is no diff available. I also added tests.

@DouweM I believe you wrote most of this code in the first place ; does this looks correct to you, or is there a better way to resolve this issue maybe?

See merge request !3135

Changing filter dropdowns shows loading

Instead of doing a full refresh of the page - i've modified the filterResults method on the Issues object to work for this form as well

![issues](/uploads/3335f09f7df88b6d419de7fd3d6857d2/issues.gif)

Closes #14359

See merge request !3237

Add confidential issues

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/3678

More information: https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/227

See merge request !3282

Show a notice for diffs that are empty/too large

This builds upon https://gitlab.com/gitlab-org/gitlab_git/merge_requests/72 but will fail until said MR has been merged.

Fixes gitlab-org/gitlab-ce#13912, gitlab-org/gitlab-ce#13644, gitlab-org/gitlab-ce#13557, gitlab-org/gitlab-ce#13267, gitlab-org/gitlab-ce#3906

See merge request !3185

[ci skip]