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.
Fix feedback message display
Merge pull request #52 from mtyaka/assessment-patch

Keep submit button disabled after answering in assessment.
display-name: Update usage of `display_name` to include upstream changes

It used to be on a separate proeperty - it's now using a real XBlock
field and its default value
Merge pull request #50 from edx-solutions/display-name

display-name: Update usage of `display_name` to include upstream changes
Merge pull request #53 from aboudreault/assessment-fixes

Fix feedback message display
Some fixes for assessment workflow..
Merge pull request #54 from aboudreault/assessment-fixes

Some fixes for assessment workflow..
Update and expand the README file.