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.
a) The idea was to write to session only when the user is authenticated. And user available only in process response.
b) We can store two values ip address and location. And if IP is changed then recalculate location. @cpennington what do you think?
c) We need to decide about embargo to make it in this pr or in the next. 

I prefer to make this for general use. It seems useful. 
If you have no concerns in performance then it possible:1) to enable this middleware globally not per-view; 2)move to process_request;3)store ip and location in session, recalculate location if ip is changed. OK?

In future we can move location to user information service.