1. Make the "subscribed" text in Issuable sidebar reflect the labels
   subscription status

2. Current user mut be logged-in to toggle issue/MR/label subscription

1. Allow subscribing (the current user) to a label

- Refactor the `Subscription` coffeescript class
  - The main change is that it accepts a container, and conducts all
    DOM queries within its scope. We need this because the labels
    page has multiple instances of `Subscription` on the same page.

2. Creating an issue or MR with labels notifies users subscribed to those labels

- Label `has_many` subscribers through subscriptions.

3. Adding a label to an issue or MR notifies users subscribed to those labels

- This only applies to subscribers of the label that has just been
  added, not all labels for the issue.

This reverts commit 01160fc0, reversing
changes made to 4bff9daf.

Also allowed issues to be bulk unassigned

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

Changed breakpoint class into singleton

This class checks the current bootstrap breakpoint

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

Moved colours to variables

Closes #13854

Closes #13878

Closes #14195

Fixes #13814

Fixes an issue where values couldnlt be unselected