Resolve "Repository mirroring should notify when failed"

What does this MR do?

When a mirror gets hard failed a notification should be sent to the mirror_user to let him know that the mirror has been paused for indefinite time

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

Why was this MR needed?

Screenshots (if relevant)

HTML version

Plain text version

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
  • Has been reviewed by Backend
  • Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together
• Internationalization required/considered
• If paid feature, have we considered GitLab.com plan and how it works for groups and is there a design for promoting it to users who aren't on the correct plan
• End-to-end tests pass (package-qa manual pipeline job)

What are the relevant issue numbers?

Closes #1123 (closed)