Add Monitor / Health Check for Celery Service

[rajadain]

As observed in the fix for #3593 and #3594, there are times when the celeryd service doesn't start, or crashes for whatever reason. Whenever this happens, we should be notified.

Add a monitoring message that checks if the Celery service is down. If so, it should notify Slack. It should also try to restart the service, and report if successful or not.