Add pointer tag OLX format support in the WordCloud and LTI XBlocks

[ttqureshi]

• resolves: Add support for pointer tag support in the WordCloud and LTI XBlocks public-engineering#441

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[ttqureshi]

This PR is up for review

cc: @openedx/axim-aximprovements

[kdmccormick]

I know that I'd originally asked that XmlMixin not be brought in from edx-platform. When I asked that, I didn't realize how much code would need to be brought into each individual block in order to support pointer tags. This would be a lot of duplicated code.

@ttqureshi , could you refactor this so that the shared methods are in a mixin, just like they were in edx-platform? Call it LegacyXmlMixin, and add a docstring clarifying that it's a temporary solution and that we don't want new blocks to use the mixin.

[ttqureshi]

@kdmccormick
Alright! Let me go ahead with this approach. It sounds more reasonable to have LegacyXmlMixin as a shared utility. I'm going to add this mixin in the xml_utils file and all our extracted XBlocks will extend from this. And yeah, I'll mention in the docstring that it's a temporary solution, we will remove it in the future improvements.