Support epic subscriptions and email notifications
- Add the notifications “widget” to the epic sidebar, which allows users to subscribe/unsubscribe
- Apply the same auto-subscription rules from other issuables (mentioning someone subscribes them, etc.)
- Send out email notifications like other issuables
- You should not get email notifications if you don't have access to the epic, as @smcgivern mentions in https://gitlab.com/gitlab-org/gitlab-ee/issues/4323#note_51283033, even if you are mentioned. This is consistent with behavior for issues and merge requests.
- None yet