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.
Not a good reason! The reason why I used an array instead of a dict was I didn't know what key I should use in a dict. There's still a little ambiguity in my mind - filename seems right until you think about filenames with identical names uploaded from different directories.

Anyway, I had planned to make a final decision when I added support for paging/sorting. At that point, I'll likely switch to a dict - I suspect it'll make that work easier. Do you think I should go ahead and do so?