Give SINGLE_USER a chance to register (#1820)

An attempt to open a brand new Mastodon instance configured
as SINGLE_USER_MODE=true will cause an exception.

Enable temporary registration if we have no users in the database

Fixes #1817

app/controllers/auth/registrations_controller.rb

@@ -28,7 +28,7 @@ class Auth::RegistrationsController < Devise::RegistrationsController
   end
 
   def check_enabled_registrations
-    redirect_to root_path if Rails.configuration.x.single_user_mode || !Setting.open_registrations
+    redirect_to root_path if single_user_mode? || !Setting.open_registrations
   end
 
   private