Grading should be a separate table

Lots of coupling currently; by right unsubmission should reset the state (remove XP, remove submitted at, etc) but we also want to keep the grading comments (attached to the answer version)

At the same time, there's also a "grader ID" null/not-null in the answer table which is used for `graded_count`

Need to come up with a design (possibly with something like an answer "version")