Fix new post notifications not being sent

It seems that f50122a actually caused
a regression. With the order of execution of extensions, this event
listener was always being executed before the discussion metadata
was updated, and the listener priority was not actually necessary.

Fixes flarum/framework#1646

src/Listener/SendNotificationWhenReplyIsPosted.php

@@ -69,7 +69,7 @@ public function whenPosted(Posted $event)
         $notify = $discussion->readers()
             ->where('users.id', '!=', $post->user_id)
             ->where('discussion_user.subscription', 'follow')
-            ->where('discussion_user.last_read_post_number', $discussion->last_post_number - 1)
+            ->where('discussion_user.last_read_post_number', $discussion->last_post_number)
             ->get();
 
         $this->notifications->sync(