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.
Please add an `else` clause that returns `None`, rather than relying on falling through the end of the method. It makes it clearer what all of the values/cases for returning are.