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.
@jarv can we use this like other settings that are set in json? So we can read "Enable_video_CDN" from json? So it will enabled only for edX instances?