- Saves sent posts during rendering by sending queue worker

- Prevents empty notification emails from being sent
- Hooks to WP's post update and rewrite post notification logic
- Prevents scheduling multiple queues of the same newsletter
- Fixes issue with segments not updating when scheduling a newsletter
- Removes depreciated hash field & associated logic

lib/Models/Subscriber.php

@@ -167,7 +167,10 @@ class Subscriber extends Model {
         }
 
         // welcome email
-        Scheduler::welcomeForSegmentSubscription($subscriber->id, $segment_ids);
+        Scheduler::scheduleSubscriberWelcomeNotification(
+          $subscriber->id,
+          $segment_ids
+        );
       }
     }