Stanford-only: fix anonymous access with the grade me button

Deny self-service certificates for anonymous users
    
Bugfix for the following condition:
When anonymous (e.g., non-registered) access is enabled for a course, and that course also uses the /request_certificate endpoint (e.g., via the "Grade Me" button), anonymous users could request to be graded and given certificates.
    
As a matter of policy, Stanford doesn't want to give certificates to anonymous students.

This will probably need to be tested on stage before merge.

@jbau plz