Provide a better way to monitor/reset the state of mirror scheduling
In the past week, `RepositoryUpdateMirror` jobs ran away to 500,000 jobs in the queue. There's no good way to drop this queue and reset the state of the worker.
I don't like the idea of a button to do this, but the current mode of resetting the state of the mirrors is not a good one. This is what we have to do in production on the Rails console:
```
curl -O /tmp/sq.rb https://gitlab.com/gitlab-com/runbooks/raw/master/troubleshooting/db_scripts/sq.rb
```
```ruby
load '/tmp/sq.rb'
PULL_CAPACITY_KEY = 'MIRROR_PULL_CAPACITY'
100.times { kill_jobs_by_worker_name(OpenStruct.new(dry_run: false), 'RepositoryUpdateMirrorWorker'); sleep 0.01 }
Gitlab::Redis.with { |redis| redis.del(PULL_CAPACITY_KEY, projects) }
```
/cc: @tiagonbotelho, @DouweM, @pcarranza
issue