Adds API to fetch all legacy library content blocks that are ready to be updated to use library v2 and convert to item banks.
Also adds API to update all the references via a user celery task and to fetch its status.
* feat: add xblock endpoint for updating an xblock
fix: remove debugger
feat: make function call more generic
refactor: just use request.json for request data as before
refactor: extract method
fix: revert wrong method change
fix: refactor correct method
feat: use handle_xblock method so that we can do more than update xblocks
fix: usage_key_string defaults to None
add all CRUD operations
fix usage key parameter
refactor: create /views folder
refactor: move xblock view functions to xblock_services
fix: tests
fix: tests
refactor: move xblock API endpoint to contentstore
* docs: add explanatory comment to new xblock_service
* feat: add feature flag for enabling content editing api
* feat: raise 404 if studio content api is disabled
* tests: test xblock endpoint
* test: make all post tests work
* test: check that xblock_handler receives correct args
* refactor: create util mixin for course factories with staff
* refactor: extract course staff authorization tests
* refactor: extract tests to api view testcase class
* test: add get tests
* test: fix tests
* test: fix tests
* test: fix tests
* test: add all crud tests
* fix: refactor to fix tests
* fix: merge conflict
* fix: merge conflict
* fix: tests after merge
* fix: json request decorator
* fix: lint
* fix: lint
* fix: lint
* fix: lint
* fix: new test files
* fix: lint
* fix: lint and apply PR suggestions
* fix: lint
* fix: lint
* fix: lint
* fix: lint
* fix: lint
* fix: lint
