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.
Ah makes sense. In that case I would lean towards post-delete since the only thing we're accessing is the database name. As mentioned above, it also ensure that there are no outstanding operations to be performed on the scenario database before it gets wiped.