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.
I would set the default to `None` (and add a comment about what it's used for). Also, it might be worth setting this up as a mapping of `'CN': 'http://api.xuetangx.com/edx/video?s3_url='`, so that it's obvious that it's only mapping users from china, not all users.