Phase 1 of adding asset metadata saving to old Mongo

@dmitchell @cpennington Here's the first phase of my work. Look at the tests to see what is implemented and what isn't yet.
Do you have a default behavior which works if this isn't provided? You'll need to put a ticket into devops to have them add this to their yaml for the production server configs and you'll need to let the edx-code community know about it.