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.
@olmar Do we really need to add this url to `XuetangX`'s CDN? 
I'd prefer to set  `None` by default and guys from XuetangX may set it on their platform by themselves. Or we want to use `XuetangX` globally for all open-source edX platforms?
@Lyla-Fischer @cpennington ^^