Mobile/studio video upload

https://openedx.atlassian.net/browse/MA-104
@andy-armstrong please review.  This is the initial PR for the video upload pipeline work.
@gwprice please review.
@doctoryes FYI - first usage of the AssetMetaData store.

This is consistent behavior with the rest of Studio endpoints.
Also, unlike LMS, where course enrollment leakage would occur, in Studio, this permission error doesn't imply non-enrollment.