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/logical/post_sets/post_relationship.rb

@@ -1,24 +0,0 @@
-module PostSets
-  class PostRelationship < PostSets::Post
-    attr_reader :parent, :children
-
-    def initialize(parent_id, options = {})
-      @parent = ::Post.where("id = ?", parent_id)
-      @children = ::Post.where("parent_id = ?", parent_id).order("id ASC")
-      if options[:include_deleted]
-        super("parent:#{parent_id} status:any")
-      else
-        @children = @children.where("is_deleted = ?", false)
-        super("parent:#{parent_id}")
-      end
-    end
-
-    def posts
-      @parent + @children
-    end
-
-    def presenter
-      ::PostSetPresenters::Post.new(self)
-    end
-  end
-end