Create process for clearing out old Submission records

[ssciolla]

As of now, there is no formalized approach for clearing out old submission from the MySQL database. Data probably only needs to be kept for the current and most recent term. We should create some transparent process that allows us to do this consistently without custom SQL queries. This could probably take the form of a management command and a conditional controlled by an environment variable in start.sh. The management command (or the underlying process it uses) could have a default, accept arguments, or be controlled by configuration, and would likely use the Django ORM to filter by Submission.graded_timestamp and then bulk_delete.