Return empty values and log a warning instead of raising NotImplemented
exceptions for XML modulestore asset metadata methods.
Export modulestore-stored asset metadata as XML to exported course.
Import asset metadata XML into modulestore.
Optimize importing many items of asset metadata by avoiding multiple
round-trips to MongoDB.
Add wrapper class that enables storage of Mongo document id separately
from dictionary of all asset metadata by asset type.
Merge pull request #6180 from edx/jeskew/import_export_asset_xml_pt_2

Import/export modulestore-stored asset metadata as XML from/to exported course.