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 think lets enable the middleware globally, but store the IP in the session and only recalculate if the ip has changed. That avoids any unnecessary calls to pygeoip, but still makes the resulting process much more uniform.

I think changing the embargo code in a separate PR makes. sense.