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.
They actually will *not* compare equal. *BUT* only because the `asset_type` in the `AssetLocator` of each object will be different ('thumbnail' vs. 'asset'). I'm going to add a check on that `asset_type` in each object to make sure it's the expected value.