Deploy Celery workers for Appointment

[ryanjjung]

Background

Appointment demands some background workers. We'll do that with Celery. Some of this has landed in local dev already, but we need to get prepared for deployment in our live environments.

Deployment Details

For each of stage and prod:

• Deploy a new AutoscalingFargateCluster for this purpose.
• Install celery and flower services using the images made for Celery to run in.
• Set up communication between the publisher (the Appointment containers), the consumer (Celery), the monitor (Flower), and the Redis cluster they all talk over. Use a custom channel for communication so there is no clobbering with other uses Appointment has for Redis.