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.

I was only non-gentle because I knew it was copied from elsewhere. We can probably live with this for now, but I wanted to raise the issue so we can figure out how to do better in the future.