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
https://github.com/edx/edx-platform/pull/20645
This introduces:
* A new XBlock runtime that can read and write XBlocks that are persisted using
Blockstore instead of Modulestore. The new runtime is currently isolated so
that it can be tested without risk to the current courseware/runtime.
* Content Libraries v2, which store XBlocks in Blockstore not modulestore
* An API Client for Blockstore
* "Learning Context" plugin API. A learning context is a more abstract concept
than a course; it's a collection of XBlocks that serves some learning purpose.
