Fix regressions related to deleting posts

- On the front-end, correct the check to see if the discussion has no more posts
- On the back-end, run a query to count the posts instead of using the comments_count, because the comments_count does not include other deleted posts

framework/core/src/Core/Listener/DiscussionMetadataUpdater.php

@@ -54,7 +54,7 @@ class DiscussionMetadataUpdater
 
         $discussion = $event->post->discussion;
 
-        if ($discussion->comments_count === 0) {
+        if ($discussion->posts()->count() === 0) {
             $discussion->delete();
         }
     }