MA-135, MA-122 Support CDN, alternative sources with VAL-enabled-videos

https://openedx.atlassian.net/browse/MA-122
LMS Player: When there is no YouTube link associated with VAL, play back S3 video if available
https://openedx.atlassian.net/browse/MA-135
VAL: Support Xuetang CDN rewrite on LMS

@ormsbee reviewer.
@olmar FYI.