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Fixes #13261

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

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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.

Related to #2235.

This is done by:
1. Delaying the notification sending after the SSH key is commited in DB
2. Gracefully exit the mailer method if the record cannot be found

This reverts commit 5a586f36

This reverts merge request !3191

Closes #13805

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

This commit fixes #13334, removing an overlap between the
anchor icon and the sidebar

Fixes #13928