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

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.

Mentoring assessment mode

Add `display_name` to the Data export XBlock.

Make use of the new optional enforce_type flag

Move to use opaque keys

CSS Fixes - Additional fixes of CSS from QA

* Proper alignment when MRQ/MCQ choices span over more than one line
* Show disabled submit button upon first load
* Remove extra color from submit button
* Size of feedback title

Inspired from https://github.com/edx/edx-ora2/pull/330

Style changes

Only close the feedback popup when clicking on the close 'x' button

It used to close when any part of the popup would be clicked.

Clear results when answer is changed.

Tip overflow

When user submits the answers, the results (checkmarks/popups/tips/messages)
are displayed, and the submit button is disabled.
If user then changes any of the answers, the results are cleared and the
submit button re-enabled (but only if the answers pass validation).

Use StringIO instead of cStringIO to support Unicode in LightChildren

Design refresh fixes2

Some more work for a better styling

Design refresh - Adds extra fixes

Update display when answer is changed.

When the user first submits the answers, the checkmark/cross icons,
tip popups, and a message are shown to the user.

If the user changes some of the answers, the checkmarks, popups, and message
should automatically update. This is done by automatically resubmitting the answer.
Auto submission is only enabled if number of attempts is not limited (max_attempts = 0),