Email notifications for epics

• Support email notifications for epics, similar to issues.
• Behavior analogous to issues.
• Send email notifications, with same formatting as issues. Some more specific elements worth pointing out:
  • Change the X-GitLab-Issue-ID email header to be epic-specific
  • Change the ViewAction schema in the email to be View Epic, linking to the epic
• It should be possible to reply to the email notification, which creates a comment in the epic
• Same trigger events/conditions for sending as issues, excluding events that cannot happen on epics currently, such as assigning a user to an epic.
• Same implicit notifications (e.g. when you are @ mentioned) and explicit (when you toggle the notifications toggle in the sidebar).
• Toggle UI in the epic sidebar.
• Participants UI in the epic sidebar.
• Notifications settings follow the group settings for the group.
• Update the notifications settings API accordingly. Scope of this to be handled in issue https://gitlab.com/gitlab-org/gitlab-ee/issues/5863