- 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/Router/Endpoints/ViewInBrowser.php

@@ -10,7 +10,6 @@ class ViewInBrowser {
   const ACTION_VIEW = 'view';
 
   static function view($data) {
-    $viewer = new NewsletterViewInBrowser($data);
-    $viewer->view();
+    NewsletterViewInBrowser::view($data);
   }
 }