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
what are you disabling here? Can you instead use the name (works just as well and is clearer) eg `pylint: disable=your-dumb-error`? The name will print out in the Pylint output.