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
You could just leave the code always calling `.to_deprecated_string`. The code that @mmprandom is writing will convert that to a `unicode` call which will respect the `deprecated` status of the key, and log a warning. Once we've switched to that code, then we can convert this code over to just call `unicode` directly.