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 for forward compatibility.  I envision possibly needing to return additional data along with the videos.  I'd prefer to design APIs this way so the client always expects a dict. 
As a concrete example of an API that returns a list and not a dict, which makes it hard to update the endpoint, talk to Akiva.  He's facing this issue with one of the current mobile_api endpoints.