posts/show: refactor parent/child notices.

* Convert notices from helpers to partials.
* Eliminate PostSets::PostRelationship class in favor of post_sets/posts template.
* Eliminate COUNT(*) queries when calculating the number of child posts.
* Eliminate redundant parent load and parent exists queries.

app/models/post.rb

@@ -1239,10 +1239,6 @@ class Post < ApplicationRecord
       end
     end
 
-    def parent_exists?
-      Post.exists?(parent_id)
-    end
-
     def has_visible_children?
       return true if has_active_children?
       return true if has_children? && CurrentUser.user.show_deleted_children?