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.
@auraz if you add this setting to `aws.py`, then it can be read from json. That would also make it off by default, but configurable. You also haven't my suggestion of making it a location -> url mapping.