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.

Yes, that's true.  I went back and forth again on this after discussing with Cale.  But I've come back to what we had already decided.  So I'll add a "vid-v1:" prefix here.