Detect when redis is down; add slack alert

[jason-upchurch]

Summary

this issue is an outcome from the 20200909 postmortem dealing with Redis maintenance and download outages. We would like to be notified of outages in slack

Technical considerations

• set up poll via Pingdom?
• can we avoid caching on any checks?
• set up celery supervisor? Research adding a supervisor to celery #4573