Celery backend: Switch from Redis to Django DB

[rtibbles]

Observed behavior

We currently use a Task Model to track tasks - however, because our actual Celery backend is in Redis, it is possible for these two systems to get out of sync and to give false or misleading information to users and the rest of the system.

Expected behavior

The Task model should accurately represent the state of the task.

The easiest way to do that seems to be to use the Task model as the backend for Celery.