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.
My ideal solution would be to have the field use the scope BlockScope=Type, UserScope=None, and then have that read from an isolated part of django settings. I'm not sure if this PR is the appropriate place for that, though. (On the other hand, it might be worth just doing it so that we have the machinery in place the next time this comes up).