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.
`None` means "not able to retrieve assets" - and indicates more of an error than an empty list or dict, but less of an error than raising an exception. I've followed this pattern for everything except `get_all_asset_metadata()` in the XML modulestore which needed to return an empty list in order for export to work.