- Removes requirement of passing newsletter id when tracking clicks

- Extracts common tracking data processing/validation code into the Track
  class
- Refactors Clicks, Opens and View in Browser classes to enforce
  subscriber id and token check
- Allows admin users to preview newsletters without tracking statistics

lib/Cron/Workers/SendingQueue/Tasks/Newsletter.php

@@ -88,7 +88,6 @@ class Newsletter {
     );
     if($this->tracking_enabled) {
       $prepared_newsletter = NewsletterLinks::replaceSubscriberData(
-        $newsletter['id'],
         $subscriber['id'],
         $queue['id'],
         $prepared_newsletter