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 up the FieldData passed in to the XBlock instance read those values from something like `django.conf.settings.XBLOCK_CONFIGURATION['video']['video_cdn_url']` (for the video field named video_cdn_url)