auth: Add a method to check if an AuthUser is the default user.

rhodecode/lib/auth.py

@@ -865,6 +865,10 b' class AuthUser(object):'
         return auth_tokens
 
     @property
+    def is_default(self):
+        return self.username == User.DEFAULT_USER
+
+    @property
     def is_admin(self):
         return self.admin