Add password challenge to 2FA settings, e-mail notifications (#11878)

Fix #3961

app/models/form/challenge.rb

@@ -0,0 +1,8 @@
+# frozen_string_literal: true
+
+class Form::Challenge
+  include ActiveModel::Model
+
+  attr_accessor :current_password, :current_username,
+                :return_to
+end