Import/export modulestore-stored asset metadata as XML.

Export modulestore-stored asset metadata as XML to exported course.
Import asset metadata XML from exported course into modulestore.
Minor changes to asset XML schema definition.
It could potentially affect test instances with existing data. They'd need to wipe out their existing data -or- the Mongo data would need to be migrated with a script - a script which currently doesn't exist.

The video usage PR hasn't merged to master yet - so it wouldn't affect production data.