Add an instance-wide option to disable pull repository mirroring after n successive mirror update failures

A huge number of pull-mirror jobs on GitLab.com have not succeeded in a very long time, but continue to be processed multiple times a day.

On each attempt, we will attempt three retries, and eventually fail the job to the deadletter queue.

repository_update_mirror jobs are in fact the number one source of deadletter queue entries in Sidekiq on GitLab.com.

One possible solution would be to add an instance-wide option to disable pull repository mirroring after n successive mirror update failures.

(I am also open to other options but am starting with this proposal 😄)