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/assets/javascripts/discourse/app/models/site.js

@@ -132,7 +132,9 @@ Site.reopenClass(Singleton, {
   // The current singleton will retrieve its attributes from the `PreloadStore`.
   createCurrent() {
     const store = Discourse.__container__.lookup("service:store");
-    return store.createRecord("site", PreloadStore.get("site"));
+    const siteAttributes = PreloadStore.get("site");
+    siteAttributes["isReadOnly"] = PreloadStore.get("isReadOnly");
+    return store.createRecord("site", siteAttributes);
   },
 
   create() {

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

app/serializers/site_serializer.rb

@@ -12,7 +12,6 @@ class SiteSerializer < ApplicationSerializer
     :top_menu_items,
     :anonymous_top_menu_items,
     :uncategorized_category_id, # this is hidden so putting it here
-    :is_readonly,
     :disabled_plugins,
     :user_field_max_length,
     :post_action_types,
@@ -95,10 +94,6 @@ class SiteSerializer < ApplicationSerializer
     SiteSetting.uncategorized_category_id
   end
 
-  def is_readonly
-    Discourse.readonly_mode?
-  end
-
   def disabled_plugins
     Discourse.disabled_plugin_names
   end