FIX: Ensure UserField changes are reflected instantly in webhooks (#12291)

The Guardian object memoizes a list of allowed user fields. Normally this is fine because Guardian objects only persist for a single request. However, the WebHook class was memoizing a guardian at the class level. This meant that an app restart was required for changes to be reflected. Plus, the Guardian was being shared across all sites in a multisite instance.

Initializing a guardian is cheap, so we can manage without memoization here.

app/models/web_hook.rb

@@ -110,7 +110,7 @@ class WebHook < ActiveRecord::Base
   private
 
   def self.guardian
-    @guardian ||= Guardian.new(Discourse.system_user)
+    Guardian.new(Discourse.system_user)
   end
 end