REFACTOR: Reduce dependency on the post stream in TopicView.

This will allow us to drop the post stream from the payload for
mega-topics. On smaller topics, the extra query is fast because
of an existing index.

db/migrate/20180710075119_add_index_topic_id_sort_order_on_posts.rb

@@ -0,0 +1,5 @@
+class AddIndexTopicIdSortOrderOnPosts < ActiveRecord::Migration[5.2]
+  def change
+    add_index :posts, [:topic_id, :sort_order], order: { sort_order: :asc }
+  end
+end