Move context_managers to base class, overridden by mixed.

...angoapps/contentstore/management/commands/clone_course.py

...apps/contentstore/management/commands/migrate_to_split.py

cms/djangoapps/contentstore/tests/test_clone_course.py

cms/djangoapps/contentstore/tests/test_contentstore.py

cms/djangoapps/contentstore/utils.py

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

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

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

common/lib/xmodule/xmodule/modulestore/xml_exporter.py

common/lib/xmodule/xmodule/modulestore/xml_importer.py