Redirect Chinese students to a Chinese CDN for video.

@clytwynec 
@Lyla-Fischer 

Please ask devops to add this to edxapp/lms.env.json:
```json
  "VIDEO_CDN_URL": {
        "CN": "http://api.xuetangx.com/edx/video?s3_url="
    }
```
We should also be timing these external requests.