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.

Edge cases

• None yet