Delete scenario db on Scenario model deletion

Uses a Celery job to delete a Scenario's associated database when the
Scenario object is deleted. Triggers off the Django pre_delete signal so
that the database is deleted even when deleting objects via a batch
process which skips the model's delete() function.

Most of this is just getting the signal handler wired up.
I chose to use pre-delete because in pre-delete you're guaranteed to get a known-good object. In post-delete, you get an object that has already been from the database, so you could potentially run into problems accessing foreign keys and such things (although I don't think it makes a difference here since the only field that gets used is a string).

I think you're right that there's a potential race condition though--I'm happy to switch if you have a strong preference, I'm not sure which hazard is more important to avoid.