Reset scores (null submission approach)

Responding to @ormsbee 's concerns on #267.  In this approach, "reset" scores are created with `submission` set to null, so that we can determine the student's effective score from the `Score` model alone.

The semantics of `reset_score()` are the same for either implementation, so the test suite is almost the same (except for changing where the DB exception is raised and the order of arguments to `reset_score`)

@ormsbee @stephensanchez