[webui] Don't use Configuration.first UnregisteredUser

src/api/app/models/unregistered_user.rb

@@ -18,7 +18,7 @@ def self.can_register?
       if CONFIG['proxy_auth_register_page'].blank?
         raise ErrRegisterSave.new "Sorry, please sign up using the authentification proxy"
       else
-        raise ErrRegisterSave.new "Sorry, please sign up using #{::Configuration.first.proxy_auth_register_page}"
+        raise ErrRegisterSave.new "Sorry, please sign up using #{CONFIG['proxy_auth_register_page']}"
       end
     end
     # Turn off registration if its disabled