Xblock-drag-and-drop-v2

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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

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* ([88](https://github.com/openedx/xblock-drag-and-drop-v2/pull/88)) Cleaned up studio editor template: styling, help texts, accessibility attributes, classes instead of IDs, etc.
* ([95](https://github.com/openedx/xblock-drag-and-drop-v2/pull/95)) Fixed flaky selenium tests
* ([85](https://github.com/openedx/xblock-drag-and-drop-v2/pull/85)) Accessibility improvements for item feedback popup
* ([73](https://github.com/openedx/xblock-drag-and-drop-v2/pull/73)) Accessibility improvements for item, zone and background image when using keyboard mode
* ([96](https://github.com/openedx/xblock-drag-and-drop-v2/pull/96)) Decoy items are properly accounted for in grade calculation and problem completion condition
* ([98](https://github.com/openedx/xblock-drag-and-drop-v2/pull/98)) Support for legacy item state
* ([92](https://github.com/openedx/xblock-drag-and-drop-v2/pull/92)) Prevented overlapping item placement: option to keep dropped item where learner dropped it was removed; items always use automatic layouts (left/center/right)
* ([93](https://github.com/openedx/xblock-drag-and-drop-v2/pull/93)) Per-item error feedback is shown in assessment mode when an attempt is submitted