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.
Any reason for pre_delete instead of post delete? If it's pre isn't there technically a chance that something could attempt to access the objects after this delete runs?