Ability to delete session

[awbooze]

As a TA who created a session, I would like to delete it or have it disappear when it is over.

We don't want to keep sessions forever in the database, nor do we want to have to monkey with the database every once in a while to delete old sessions. Instead, we want to have them be deleteable by the TA at the end of the session or automatically expire after the session ends. Keeping track of which user can delete which thing will require #5.