This is a new XBlock that presents a random subset of its children. As of this commit, the block
can only be added as an Advanced component. For Sumac, we plan to enable it as part of the
Libraries Relaunch Beta, under the name "Problem Bank (Beta)"
The block does not care if its children are from V1 library, V2 library, or the course itself.
It shares the randomization logic with LegacyLibraryContentBlock. It is also fully backwards-compatible with LegacyLibraryContentBlock. So, once V1 libraries are migrated to V2 libraries (after Teak), we eventually
could point the `library_content` entry point at ItemBankBlock.
Part of: https://github.com/openedx/frontend-app-authoring/issues/1385
