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
JIRA: MA-76

Also fixes checking for mobile_access when course is None
JIRA: MOB-978
Merge pull request #5920 from edx/aleffert/additional-beta-checks

Add a check for beta users in the video_info API