Closes #13628

* Add Labels tab to Groups
* Add decorator for label so it's aware of Milestones.

With this change we avoid doing N+1 queries when viewing Milestone's
Issues/MRs from a Group context.

Make methods return ActiveRecord Relations instead of Arrays.

Based on however I changed the tabindex of the search field so that it goes search field -> filter field & then back to browser default. I dont think we should override the default tabindex of the page too much
Closes #3706

This is a start on #3279 by updating the CSS for the dropdown menu. This doesnt include any CSS for the title bar or filter field

Closes #4177

Closes #13982

Prior, we were double-checking `@keys.any?`, and unnecessarily assigning
`is_admin` in a view where it wasn't needed.

Button transparent colours

Closes #13860

A file called notes is loaded every ~15 seconds which checks for
updates to content on the page. This commit increases the polling
timeout over time (15, 30, 60, 120 seconds) and resets
it to 15 seconds if the AJAX call returns new notes

Fixes issue #13300

Closes #5534

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

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