* fix: don't allow pasting xblocks with children into libraries
* fix: IntegrityError: "Column 'md5_hash' cannot be null"
* feat: allow pasting a unit from a course into a library
* feat: auto-generate a nice block_id when pasting into a library
* test: add test for pasting unit from course into library
* fix: better handle potentially missing display_names during paste
* chore: clarifications and import cleanups