Check if user is anonymous before calling PSYCHOMETRICS

LTI grade callbacks for example, come in with an anonymous user.  This causes a
stack trace in the psychometrics app that leads to the LTI service
getting a 404 when trying to grade.  This adds a check before that callback gets registered.


why not just `@patch.dict(settings.FEATURES, {'ENABLE_PSYCHOMETRICS': True})`, instead of this and also adding the overhead of two global vars?