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.  
This PR is actually defunct now.  The most recent PR for this is: https://github.com/edx/edx-platform/pull/5986

In the latest PR, we decided to actually store the created date in VAL for now.  The VAL database already has a lot of the meta-data information related to the video.  We are still using the asset-meta-data store for keeping track of which videos were uploaded from a given course.  But then the rest of the meta data (including status) is now maintained in VAL.