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.
I'll clarify the equivalence in the tests. `AssetThumbnailMetadata` objects should not be `==` to `AssetMetadata` objects - the category of each object should be different. "asset" vs. "thumbnail". The asset_ids will be different.