This is allowed for existing instances so we don't end up 76 offenses
right away, but for new code one should _only_ use this if they _have_
to remove non database data. Even then it's usually better to do this in
a service class as this gives you more control over how to remove the
data (e.g. in bulk).
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.
