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
MA-141 Support webm encoding for VAL-enabled videos
Merge pull request #5920 from edx/aleffert/additional-beta-checks

Add a check for beta users in the video_info API
Merge pull request #5981 from edx/mobile/MA-141

MA-141 Support webm encoding for VAL-enabled videos
TNL-655: Add/Edit/Remove notes.
Add cache busting.
Add platform-specific feature flag.
Fix conflicts.