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.