New admin setting: open/close registrations, with custom message, from the admin UI

app/controllers/auth/registrations_controller.rb

@@ -3,7 +3,7 @@
 class Auth::RegistrationsController < Devise::RegistrationsController
   layout :determine_layout
 
-  before_action :check_single_user_mode
+  before_action :check_enabled_registrations, only: [:new, :create]
   before_action :configure_sign_up_params, only: [:create]
 
   protected
@@ -27,12 +27,12 @@ class Auth::RegistrationsController < Devise::RegistrationsController
     new_user_session_path
   end
 
-  def check_single_user_mode
-    redirect_to root_path if Rails.configuration.x.single_user_mode
+  def check_enabled_registrations
+    redirect_to root_path if Rails.configuration.x.single_user_mode || !Setting.open_registrations
   end
-  
+
   private
-  
+
   def determine_layout
     %w(edit update).include?(action_name) ? 'admin' : 'auth'
   end