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.  
They are referenced here, and presumably could change: https://github.com/edx/edx-platform/pull/5958/files#diff-f42f588ffd4bc59c373a4ac8d260706dR179