opaque-keys: Adds support for opaque keys

Inspired from https://github.com/edx/edx-ora2/pull/330
Properly serialize OpaqueKey in light_children
pep8
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
Merge pull request #47 from edx-solutions/css-fixes

CSS Fixes - Additional fixes of CSS from QA
Make use of the new optional enforce_type flag
Merge pull request #46 from dragonfi/opaque-keys

Move to use opaque keys
Merge pull request #48 from FiloSottile/optional_enforce_type

Make use of the new optional enforce_type flag
Add `display_name` to the Data export XBlock.
Merge pull request #51 from mtyaka/dataexport-display-name

Add `display_name` to the Data export XBlock.
Mentoring assessment mode
Merge pull request #49 from aboudreault/mentoring-assessment-mode

Mentoring assessment mode
Fix typo in "assessment"
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..