feat: provisionally support V2 libraries in LibraryContentBlock (randomized only) (#33263)

Refactors and reworks the LibraryContentBlock so that its
sync-from-library operations are asynchronous and work with
V2 content libraries. This also required us to make
library_content block duplication asynchronous, as that
involves syncing from the source library.

For the sake of clarity, this PR includes two major method renames:

* update_children(...) -> sync_from_library(...)
* refresh_library(...) -> sync_from_library(upgrade_to_latest=True, ...)

an an XBlock HTTP handler rename:

  /refresh_children -> /upgrade_and_sync

There are still a couple issues with import or duplication
of library_content blocks referencing V2 libraries other than
latest. These will be resolved in an upcoming PR.

Part of: https://openedx.atlassian.net/wiki/spaces/COMM/pages/3820617729/Spec+Memo+Content+Library+Authoring+Experience+V2
Follow-up work: https://github.com/openedx/edx-platform/issues/33640

Co-authored-by: Connor Haugh <chaugh@2u.com>
Co-authored-by: Eugene Dyudyunov <evgen.dyudyunov@raccoongang.com>

docs/docs_settings.py

@@ -37,7 +37,6 @@ INSTALLED_APPS.extend([
     'cms.djangoapps.course_creators',
     'cms.djangoapps.xblock_config.apps.XBlockConfig',
     'lms.djangoapps.lti_provider',
-    'user_tasks',
 ])