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
Don't do this in `__init__`. Do this at the module level. That way it will only happen once.