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
I'm not sure I follow why you think it's outdated. Is it b/c it no longer overloads `_id` but uses the separate `content_son` field?