As a Super Admin, I want to see the success or failure of mail delivery

[camjackson]

(Issue created in core repo though it doesn't necessarily have to be there. In fact this feature probably belongs as a separate service)

1. The group-mailer currently puts out a send-email request event, with a list of recipients
2. The mailer attempts to send the email to every list recipient
3. The mailer publishes a single email-sent event containing all the recipients that succeeded
   • In case they all fail, no email-sent event is published
4. The mailer publishes a single email-failed event containing all the recipients that failed
   • In case they all succeed, no email-failed event is published

For this story, we need to aggregate all of the success and failure events, and present a UI to the super admin so that they can see, for any given email, the success or failure of email sending. Requirements aren't set in stone, but this could include:

• A list of all email campaigns, with each having a summary of e.g. "53/87 emails sent successfully"
• A click-through to each specific campaign, showing for each recipient:
  • A success icon if the email was sent to that recipient successfully
  • A failure icon if the email failed to send to that recipient
  • A pending icon if we have not received either success or failure for that recipient
• Suggest showing pending first, followed by failures, followed by success.
• Allow sorting by name/email/id to find a specific recipient

The above list is quite large, and this story probably could/show be broken down further.

From a technical point of view it's not clear where either the backend or frontend code should live. Easiest would be to pile it all into core but that might be a bad idea.