Xblock-drag-and-drop-v2

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* Enforce XBlock variable types (PR 104)
* Improvements for mobile (PRs 132, 133, 134)

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* Various accessibility improvements (PRs 110, 111, 112)

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* i18n improvements (PR 113)

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* ([108](https://github.com/openedx/xblock-drag-and-drop-v2/pull/108)) Grading updates:
* [SOL-2030][sol-2030] Grade information is now displayed beneath the display name of the problem (similar to CAPA)
* [SOL-2094][sol-2094] Fixed problem preventing DnDv2 blocks from being scored or graded.
* Highest grade achieved is now displayed in feedback area
* Renamed "Maximum score" field to "Problem Weight" to be in line with CAPA problems and avoid confusion.

[sol-2030]: https://openedx.atlassian.net/browse/SOL-2030
[sol-2094]: https://openedx.atlassian.net/browse/SOL-2094

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* ([106](https://github.com/openedx/xblock-drag-and-drop-v2/pull/106)) Don't delete misplaced items on final attempt

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* ([97](https://github.com/openedx/xblock-drag-and-drop-v2/pull/97)) Added "item" field to item.dropped event
* ([101](https://github.com/openedx/xblock-drag-and-drop-v2/pull/101)) Implement "show answer" button
* ([103](https://github.com/openedx/xblock-drag-and-drop-v2/pull/103)) Miscellaneous UI fixes
* ([105](https://github.com/openedx/xblock-drag-and-drop-v2/pull/105)) Correct an issue with background image selection