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.  
It will have the attr (it's the field definition not the value); so, that test won't work, but it may be fine as is. I didn't drill into how `video_pipeline_configured` is used. If it's just for backward compatibility, I'd rather see a real migration method rather than adding more info outside the object.