Jrbl/fixes anon grade me

@jbau this replaces the previously closed https://github.com/edx/edx-platform/pull/4065. My comments there:

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.