Use celery to schedule periodic tasks

[Iain-S]

Summary

As we add more periodic tasks (e.g. if we add usage forecasting in addition to the existing abolishments and daily emails), switching to a more widely used task management option makes sense.

Celery is a task queue, which requires a broker to mediate between clients and workers and, optionally, a backend for result storage. I have used Redis as the broker as it was easier to install than Rabbit MQ. Since we don't do anything with the results of completed tasks, I have omitted a backend but we could use Redis or Postgres if we need one in the future.

ToDo

• Fix the error that can occur if we try to insert a daily summary email for subscription UUID(int=0).
• Add Europe/London timezone?
• Shut down redis and celery on exit (particularly if using the runtests script.
• Set up Celery logging (currently logging to file)

[joseph-palmer]

🚀