FIX: Preload readonly mode attribute seperately.

There are two problems I'm trying to tackle here.

1. The site json is cached for anonymous users so readonly mode can be
cached for up to 30 minutes which makes it confusing.

2. We've already checked for readonly mode in the controller so having
to check for readonly mode again in `SiteSerializer` is adding an extra
Redis query on every request.

app/controllers/application_controller.rb

@@ -539,6 +539,7 @@ class ApplicationController < ActionController::Base
     store_preloaded("customHTML", custom_html_json)
     store_preloaded("banner", banner_json)
     store_preloaded("customEmoji", custom_emoji)
+    store_preloaded("isReadOnly", @readonly_mode.to_s)
   end
 
   def preload_current_user_data