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
Shouldn't you want to add the StudentRole here?
To verify that students (who are not BetaTesters) don't have access to courses not marked as "mobile_available".