Use anonymous user id from the runtime instead of real user id

mentoring/answer.py

@@ -105,8 +105,7 @@ class AnswerBlock(XBlock):
         if not name:
             raise ValueError, 'AnswerBlock.name field need to be set to a non-null/empty value'
 
-        # TODO Use anonymous_user_id
-        student_id = self.scope_ids.user_id
+        student_id = self.runtime.anonymous_student_id
 
         answer_data, created = Answer.objects.get_or_create(
             student_id=student_id,