Keep submit button disabled after answering in assessment.

Once a student submits an answer in assessment mode, he should not be allowed
to change it, so stop listening for changes once an answer has been submitted.

The change lister would re-enable the submit button if the student changed their
answer, which shouldn't happen in assessment mode.
Merge pull request #52 from mtyaka/assessment-patch

Keep submit button disabled after answering in assessment.