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.

Hmm.. we'll need to revisit this in our subsequent stories.  It seems we're headed in the direction of even presenting the list of video URLs for the various encodings (that is, list YouTube and S3 links).  
The reason I kept them separate for now, is because we may want to distinguish between (a) file validation completed versus (b) at least one encoding has completed.  The latter state would indicate making an extra VAL call to get the latest video encodings.