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.

Why not?  Doesn't this endpoint partially update the list of videos?
It is also consistent with many of the other endpoints in studio, at the moment.