Allow course staff to upload a course introduction video by tudou in China

Teacher can add course-introduction-video by youtube. But people in China cannot visit that website. 
Since @sarina is not sure whether edx infrastructure can handle serving the introduction video to many users
https://github.com/edx/edx-platform/pull/5119
so I open this pr to allow a video to be uploaded from tudou(www.tudou.com) provider.