Asset support in split

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

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

...e/management/commands/tests/test_rollback_split_course.py

cms/djangoapps/contentstore/tests/test_clone_course.py

cms/djangoapps/contentstore/tests/test_contentstore.py

cms/djangoapps/contentstore/tests/test_core_caching.py

cms/djangoapps/contentstore/tests/test_export_git.py

cms/djangoapps/contentstore/tests/test_import.py

cms/djangoapps/contentstore/tests/test_transcripts_utils.py

cms/djangoapps/contentstore/tests/utils.py

cms/djangoapps/contentstore/views/assets.py

...djangoapps/contentstore/views/tests/test_import_export.py

cms/djangoapps/contentstore/views/tests/test_transcripts.py

common/djangoapps/contentserver/middleware.py

common/djangoapps/contentserver/tests/test.py

common/djangoapps/terrain/browser.py

common/lib/xmodule/xmodule/contentstore/content.py

common/lib/xmodule/xmodule/contentstore/mongo.py

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

common/lib/xmodule/xmodule/modulestore/django.py