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
@cpennington I had made this change on Friday, but it was breaking tests - so I reverted the change.  I had made this change in combination with another one, so not sure which one actually broke the test.

But since this is only very short-term code (to be removed in a matter of days), I chose not to spend the time digging through it.