Make concern module more auto-load friendly

The existing folder `app/controllers/admin/concerns` is not part of the
top-level auto-load and making EE extension for this concern is
problematic. Other non-admin controller
(e.g. `OmniauthCallbacksController`) is also using this concern so it is
better place in the standard `app/controllers/concerns` folder.

The class is also renamed to be consistent with the existing counterpart
`AuthenticatesWithTwoFactor` class and avoid issue with RSpec cop
misreading the name due to usages of numbers and acronyms (i.e. `2FA`).

app/controllers/admin/sessions_controller.rb

 # frozen_string_literal: true
 
 class Admin::SessionsController < ApplicationController
-  include Authenticates2FAForAdminMode
+  include AuthenticatesWithTwoFactorForAdminMode
   include InternalRedirect
   include RendersLdapServers
 

app/controllers/admin/concerns/authenticates_2fa_for_admin_mode.rb → app/controllers/concerns/authenticates_with_two_factor_for_admin_mode.rb

 # frozen_string_literal: true
 
-module Authenticates2FAForAdminMode
+module AuthenticatesWithTwoFactorForAdminMode
   extend ActiveSupport::Concern
 
   included do

app/controllers/omniauth_callbacks_controller.rb

 # frozen_string_literal: true
 
 class OmniauthCallbacksController < Devise::OmniauthCallbacksController
-  include AuthenticatesWithTwoFactor
-  include Authenticates2FAForAdminMode
+  include AuthenticatesWithTwoFactorForAdminMode
   include Devise::Controllers::Rememberable
   include AuthHelper
   include InitializesCurrentUserMode