Set packs on 2FA-related pages. Fixes #271.

Specifically, this commit:

- changes S::TFA::{Confirmations,RecoveryCodes}Controller to derive from
  S::BaseController, because this gives us the necessary actions and
  packs
- prepends set_pack to Auth::SessionsController's action chain so that
  it takes effect in time for render :two_factor

app/controllers/settings/two_factor_authentication/confirmations_controller.rb

@@ -2,11 +2,7 @@
 
 module Settings
   module TwoFactorAuthentication
-    class ConfirmationsController < ApplicationController
-      layout 'admin'
-
-      before_action :authenticate_user!
-
+    class ConfirmationsController < BaseController
       def new
         prepare_two_factor_form
       end