Move to rabbitmq as broker

[madprime]

Following on advice from @beaugunderson in slack, who finds that RabbitMQ is generally more robust as a message broker. It seems like Redis unreliability is likely behind the hacky solution taken here: https://github.com/OpenHumans/open-humans/pull/1054 (It would be better to fix the underlying problem!)

Also mentioned here https://github.com/arvkevi/openhumansimputer/issues/38 RabbitMQ message broker + Redis results backend seems to be a common setup for celery, and that's what ended up working for @arvkevi.

[madprime]

@gedankenstuecke any comments on this? It's not high priority, but would be great to yes/no for the record. Thanks!

[gedankenstuecke]

Ah, yes! Agree on that switching would be good as Redis seems unreliable!