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.

The PUT method generally means "replace the thing at this URI with the thing I'm sending," which is not appropriate for this endpoint.