Drag and Drop v2 - Review for release on edx.org

"Show Problem heading" checkboxes.

This is to ensure compatibility with screen readers: When transforming
text to uppercase via CSS, (some) screen readers spell out individual
letters of zone labels (instead of reading labels as a sequence of words).

cf. https://github.com/edx-solutions/xblock-drag-and-drop-v2/pull/34#issuecomment-174032333

be interacted with, and doesn't intercept keyboard input.

https://github.com/edx-solutions/xblock-drag-and-drop-v2/pull/48.

While working on getting the Drag and Drop v2 XBlock ready for edx.org,
we discovered a bug in xblock-utils that kept resources containing
non-ASCII chars from being loaded correctly. This bug was fixed in
https://github.com/edx/xblock-utils/pull/36, and the fix was integrated
into edx-platform via
https://github.com/edx/edx-platform/pull/11282 (target branch: named-release/dogwood.rc) and
https://github.com/edx/edx-platform/pull/11283 (target branch: master).

Mobile support: Improve draggable width options for more flexible authoring

Fix item duplication bug that can occur (due to virtual dom change tracking mixups and jQuery UI draggable clone)

Update screenshots

DnDv2 exercises.

# Conflicts:
#	tests/integration/test_interaction.py

Ensure correct alignment of zone labels; remove IE9 shims

zone labels from LMS rules for <p> elements.