In 8278b763d9 the default behaviour of annotation
has changes, which was causing a lot of noise in diffs. We decided in #17382
that it is better to get rid of the whole annotate gem, and instead let people
look at schema.rb for the columns in a table.
Fixes: #17382
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.
