Clear results when answer is changed.

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).
tip-overflow: Add scrollbar to feedback popup & allow height overflow
Merge pull request #43 from edx-solutions/tip-overflow

Tip overflow
Merge pull request #42 from mtyaka/clear-results

Clear results when answer is changed.
Write 'submissions' instead of 'attempts'
popup-click-close: Only close the popup when clicking on the close 'x'

It used to close when any part of the popup would be clicked.
Merge pull request #44 from edx-solutions/popup-click-close

Only close the feedback popup when clicking on the close 'x' button
Change Feedback title color to #333333
Grey out disabled submit button
Merge pull request #45 from dragonfi/master

Style changes
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