Notification e-mails for failed mirror updates can spam frequently

Today @tommy.morgan, @ibaum, and I started receiving a notification e-mail that notified us that the remote mirrors were failing. I think https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/23595 was deployed in RC4. The problem is that it spams us every few minutes when this failure happens:

This can easily happen when remote mirrors fail to push due to protected branches, and a manual reconciliation is needed.

I think we should only send an e-mail the first time this problem happens and reset the flag once mirrors start function again.