Previous methods of "deleting state" for a student had just issued
reset scores, so the (unscored) submission was still hanging around.
With this change, the submission can be "deleted" for all django-
relevant purposes, while still remaining in the database in case
it turns out to be interesting to analytics later.

Includes a migration that may take some time to run, and will acquire a
table lock. This will cause an outage for any other request trying to
hit the submissions table during the migration. We've estimated the outage
to last approximately 3 minutes on a database with 700,000 rows.

Test also included.