As an Admin, I should be able to send System Emails

(User Story, To be Fixed , Priority: Critical, Test Status: No automated tests yet , Reported By Justin du Coeur, )
Email
Summary: This existed once upon a time, but has been long since removed. Now that we have Unsubscribe, re-add it.
This is a small Epic, comprising:
  • Add a new UI for Compose System Email. This should include the requirement to view and approve a proofed copy of the actual sent email, which will take a bit of refactoring. (But that latter could become a subsequent story.)
  • Make sure that the machinery for sending a Notification to all Users is working. Review it, and maybe rewrite it to be Slick-based and properly streaming.
  • Implement the SystemUpdateNotifierEcot, along the lines of InvitationNotifierEcot.
  • Implement Unsubscribe properly for this! Deathly important before the first one goes out.