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.
@cpennington  What about if we will create hidden course field. And settings.VIDEO_CDN_URL will be read into that course field on course instantiation. Then, we can use this field by inheritance. What you think?