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

Export modulestore-stored asset metadata as XML to exported course.
Import asset metadata XML into course's modulestore.
Return empty values and log a warning instead of raising NotImplemented exceptions for XML modulestore asset metadata methods.
Optimize importing many items of asset metadata by avoiding multiple round-trips to MongoDB.
@dmitchell  This change fixed a bug found in the `test_copy_all_assets_from_course_with_no_assets` test added below - fyi.