Enable cross-modulestore copying of course asset metadata.

https://openedx.atlassian.net/browse/PLAT-38
I'm changing them to to_storable/from_storable. It's not JSON, it's not BSON, it's a Python structure that's able to be stored in a MongoDB. There's no easy name for that format - so I'm sticking with a general name that says nothing about the format.