Xblock-drag-and-drop-v2

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* Add package publishing workflow.

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* Make the "Show Answer" condition customizable (like in the Problem XBlock).

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* Support HTML in explanations.

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* Remove dots on feedback and moves final feedback to end and set bold text on assessment mode.
* Improve CSS styles for popup feedback.
* Remove special popup feedback styles for mobile.
* Improve feedback message when you reach the limit of elements on a zone.
* Fix the issue while dragging an image item on Safari mobile.
* Fix width issue for zone container on Safari mobile
* Update translations.

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* Drop the support for Python 3.5 and Ubuntu 18.04.

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* Use global (platform-wide) translations as a fallback when XBlock translations are missing. \
This change also has an intended side effect - it allows overriding English text with comprehensive themes (by patching the default translation catalog).