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/application_controller.rb

@@ -8,7 +8,7 @@ class ApplicationController < ActionController::Base
   force_ssl if: "Rails.env.production? && ENV['LOCAL_HTTPS'] == 'true'"
 
   include Localized
-  helper_method :current_account
+  helper_method :current_account, :single_user_mode?
 
   rescue_from ActionController::RoutingError, with: :not_found
   rescue_from ActiveRecord::RecordNotFound, with: :not_found
@@ -69,6 +69,10 @@ class ApplicationController < ActionController::Base
     end
   end
 
+  def single_user_mode?
+    @single_user_mode ||= Rails.configuration.x.single_user_mode && Account.first
+  end
+
   def current_account
     @current_account ||= current_user.try(:account)
   end