Resolve "Email notifications for epics"

What does this MR do?

Support email notifications for epics, similar to issues.

TODO:

• 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.
  • send when participant
  • send when mentioned
• Toggle UI in the epic sidebar.
• Participants UI in the epic sidebar.
• Notifications settings follow the group settings for the group.

Are there points in the code the reviewer needs to double check?

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Tests added for this feature/bug
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
  • Has been reviewed by Backend
• Squashed related commits together

What are the relevant issue numbers?

Closes #5480 (closed)