FIX: Clear anon cache when disabling readonly mode.

`SiteSerializer#is_readonly` is cached for an anonymous user so we have
to clear the cache when disabling readonly mode. Otherwise, the site may
appear to be in readonly mode for an extended period of time.

spec/components/discourse_spec.rb

@@ -190,11 +190,17 @@ describe Discourse do
     describe ".disable_readonly_mode" do
       it "removes a key from redis and publish a message through the message bus" do
         Discourse.enable_readonly_mode
+        message = nil
 
-        message = get_readonly_message do
+        messages = MessageBus.track_publish do
           Discourse.disable_readonly_mode
         end
 
+        expect(messages.any? { |m| m.channel == Site::SITE_JSON_CHANNEL })
+          .to eq(true)
+
+        message = messages.find { |m| m.channel == Discourse.readonly_channel }
+
         assert_readonly_mode_disabled(message, readonly_mode_key)
       end