Generalize assets to any block_type but using

AssetKey and the metadata store
PLAT-204

@cpennington @doctoryes plz review (hopefully I'll fix the 2 typos I just noticed before you get to it)
What if ```_id``` is not present as a key? Does this mean that _find_course_assets() always returns the assets along with the _id? Maybe ```_id``` should be removed upstream?