Split mongo assets cherry

Asset support in split
LMS-2876

Kind of, because really using in split requires the opaque url refactoring and CourseLocator.make_asset_key, this actually doesn't do anything with split, but it gets close.

@nasthagiri @cahrens @dianakhuang please review
alas, `make_asset_key` is implemented to throw `NotImplementedError`. I changed it to look in the `KEY_FIELDS` list.