Dedupe labels in labels selector in Dashboard pages

Fixes #14418.

See merge request !3311

Also moved useless helper methods and directly to the
DashboardController.

Do not retry "git gc"

To prevent 'git gc' timing out on a large repo and then bouncing
around in the retry queue.

See merge request !3266

Fix an issue when the target branch of a MR had been deleted

Before displaying the "diverged commits" note, we're checking if the MR
is open, but we should check if it's mergeable instead because this
check ensure the source and target branches exist.

This was introduced by !2217 and fixes #14388.

See merge request !3294

Refactor colors and lists

* Introduce 2 color links: black and blue
* Use black color link for UI elements like lists
* Refactor lists to use .title nested under li
* Make all lists (events, todos, activity, projects etc) use style
* List colorschema is now 333 for links and 555 for text
Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

cc @jschatz1 @skyruler 

See merge request !3307

* Introduce 2 color links: black and blue
* Use black color link for UI elements like lists
* Refactor lists to use .title nested under li
* Make all lists (events, todos, activity, projects etc) use style
* List colorschema is now 333 for links and 555 for text
Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

We have to ensure source_sha and target_sha are not nil before calling
Gitlab::Git::Commit.between.

Before displaying the "diverged commits" note, we're checking if the MR
is open, but we should check if it's mergeable instead because this
check ensure the source and target branches exist.

This was introduced by !2217 and fixes #14388.

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]