Merge pull request #6033 from edx-solutions/content-libraries

common/djangoapps/xmodule_django/models.py

common/lib/xmodule/setup.py

common/lib/xmodule/xmodule/library_root_xblock.py

common/lib/xmodule/xmodule/modulestore/__init__.py

common/lib/xmodule/xmodule/modulestore/mixed.py

common/lib/xmodule/xmodule/modulestore/mongo/base.py

...dule/modulestore/split_mongo/caching_descriptor_system.py

common/lib/xmodule/xmodule/modulestore/split_mongo/split.py

...ib/xmodule/xmodule/modulestore/split_mongo/split_draft.py

common/lib/xmodule/xmodule/modulestore/tests/factories.py

...n/lib/xmodule/xmodule/modulestore/tests/test_libraries.py

...odule/xmodule/modulestore/tests/test_mixed_modulestore.py

common/lib/xmodule/xmodule/modulestore/tests/test_mongo.py

common/lib/xmodule/xmodule/modulestore/tests/utils.py

requirements/edx/github.txt