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
Should the default state of `'is_draft'` be `False` here? It seems like if it's unset that might be a bug that will silently succeed here.