This PR limits the number of XBlocks which can be a part of content libraries, to ensure predictability and avoid problematic load issues. It defaults to 1000 blocks limit right now, which can be configured if needed.
This PR changes a blockstore based API, which is not part of the devstack yet. To test this:
From the blockstore directory, run `make testserver` to start a test-specific instance.
From `make studio-shell` run: `EDXAPP_RUN_BLOCKSTORE_TESTS=1 python -Wd -m pytest --ds=cms.envs.test openedx/core/djangoapps/content_libraries/tests/`
[ ] @bradenmacdonald
[ ] edX reviewer[s] TBD