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
Sorry, I didn't know about that behavior of `to_deprecated_string` and for some reason, Cale's first comment wasn't showing up when I was looking for comments.