Enable cross-modulestore copying of course asset metadata.

https://openedx.atlassian.net/browse/PLAT-38
This line really shows the difficulty of calling these methods `to_mongo` and `from_mongo`. In particular, it's confuses the fact that the source store might be an `XmlModuleStore`, and given the nomenclature of OldMongo and SplitMongo, saying `mongo` for a storage format is somewhat misleading.