Redirect Chinese students to a Chinese CDN for video. BLD-1052.

Use Chinese Video CDN when users coming from China.
Issue: https://edx-wiki.atlassian.net/browse/BLD-1052

@auraz please review.
@jarv Hi, we want to use that functionality for edx.org instance, but do not touch open-source edX platforms. Can we use environment variables to enable this functionality? Other thoughts?