Adds LegacyXmlMixin in the xml_utils and make HTML XBlock use the corresponding XML parsing methods

[ttqureshi]

• resolves: Create xml_utils and add support for pointer tag utilities in the HTML XBlock. public-engineering#435

In this PR, I have:

• added the LegacyXmlMixin in xblocks_contrib/common/xml_utils.
• removed the XML parsing methods from HTML XBlock by extending it from LegacyXmlMixin.

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[kdmccormick]

LGTM!

For the other blocks, feel free to update them all in a single PR if that's easier for you.

[farhan]

Great work 🌟

[ttqureshi]

For the other blocks, feel free to update them all in a single PR if that's easier for you.

@kdmccormick thanks for the PR review. I'm currently working on the test case failures in the test PR on edx-platform. Once all checks are green we are good to merge this PR.

[ttqureshi]

@farhan
PR is up for final pass.