Adds `#build_notification_recipients` to `NotificationRecipientService`
that returns the `NotificationRecipient` objects in order to be able to
access the new attribute `reason`.
This new attribute is used in the different notifier methods in order to
add the reason as a header: `X-GitLab-NotificationReason`.
Only the reason with the most priority gets sent.
We use bcc here because we don't want to generate this emails for a
thousand times. This could be potentially expensive in a loop, and
recipients would contain all project watchers so it could be a lot.
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.
