Add a check for beta users in the video_info API

Discovered a case I missed on the original version of this. See
https://github.com/edx/edx-platform/pull/5798

@gwprice @ormsbee 

JIRA: MA-76
Maybe I misunderstand or missed it, but when I was writing this I went looking for something like that and I don't think it's a role (see student/roles.py). As far as I can tell, just adding someone to a course is the implicit student role.