Blockstore and all of its (experimental) functionality has been replaced with
openedx-learning, aka "Learning Core". This commit uninstalls the now-unused
openedx-blockstore package and removes all dangling references to it.
Note: This also removes the `copy_library_from_v1_to_v2` management command,
which has been broken ever since we switched from Blockstore to Learning Core.
Part of this DEPR: https://github.com/openedx/public-engineering/issues/238
* feat: tests for copying units in content_staging app
* chore: convert old tests to pytest style
* feat: make pasting units work (in Studio backend API)
* refactor: make XmlMixin.parse_xml more like standard XBlock behavior
