MA-104 Studio backend for video_upload

https://openedx.atlassian.net/browse/MA-104
Advanced Setting for "Video Upload Pipeline".
Feature Flag for "ENABLE_VIDEO_UPLOAD_PIPELINE".
Use AssetMetaData store to persist video upload information.

@andy-armstrong @gwprice I have created this new PR to reflect only the initial backend studio support of our feature.  Please review again.  
However, looking at the error codes used `user.py` in this module, it seems the convention is to use 400 for these situations.
https://github.com/edx/edx-platform/blob/master/cms/djangoapps/contentstore/views/user.py#L131