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.
This middleware seems odd to me.

a) Why is this in process response, rather than process request?
b) Why store the country in the session? We don't expire sessions, so that could end up being very long-lived. Maybe that doesn't matter, but I wonder about people who travel.
c) We do similar logic in the embargo middleware. It would be nice if the embargo middleware would use the data recorded by the geoinfo middleware.

If this middleware is intended just for use on a per-view basis, then why not make it a regular decorator instead. If it's intended for general use, then it seems like it should set the data before the request is processed, rather than after.