WR-05: Confirm clamps the grader's score to the question's current
points, so lowering a question's points no longer lets a confirm store
more than full marks, and Totals caps the percentage at 100 when a
question's points were raised after the attempt was scored.
- migration: open question type, rubric, max_length, quiz grader and
the grading and review columns of attempt answers
- open answers are checked, stored as pending and never scored on
answer; a changed text resets the grading state
- student JSON gains text, grading_status, grader_feedback,
review_comment, max_length, needs_review and open_answers; the
rubric is never serialized
