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.  
In that case, the AWS settings should not be in the video upload pipeline section of the settings module. Also, I'm surprised that these are not already present in common.py. Is there some reason for that that we are missing?