Setup RabbitMQ + Celery for async task handling

[batpad]

Opening up a draft PR here, still need to do some cleanup.

This:

• Adds celery, rabbitmq and django-celery-results to requirements for task queue, job processing, and reporting of Tasks in django admin
• Adds a simple Hello task to test everything is functioning correctly
• Adds instructions in README to install RabbitMQ, run Celery worker, etc. (needs some cleaning up)

@willemarcel could you give this a first pass and let know if the general approach looks good to me? I'll do a bit of cleanup - I used AI help and the README is too verbose, I'll clean that up a bit.

Separately, I might try and make things work with a .env file so one doesn't need to make sure the env vars are exported in every terminal, especially since we now need to run multiple things with the celery worker, etc.

Also, I think I might have accidentally committed some local changes I had to settings to get things to work for me - I'll clean those up as well.

[willemarcel]

@batpad sorry for the delay. The structure looks good. I think we can clean up what is not needed.

The tests are not succeeding for me. I think it's okay to isolate some functions used by the tasks, test then, and then don't trigger celery tasks on the tests.