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.
Would it be better to change it so, because 2XX-level HTTP status code is success?
```python
if 200 =< cdn_response.status_code < 300:
 ...
```