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
When you clone a mongo course with Private content to mongo, it currently fails.  This issue was introduced with our mixed MS PR.  Here's the JIRA report for this: 
https://openedx.atlassian.net/browse/STUD-1946