PERF: Add index_topic_id_percent_rank_on_posts.

Speeds up the `Posts#summary` mode query for large topics.

db/migrate/20180621013807_add_index_topic_id_percent_rank_on_posts.rb

@@ -0,0 +1,9 @@
+class AddIndexTopicIdPercentRankOnPosts < ActiveRecord::Migration[5.2]
+  def up
+    add_index :posts, [:topic_id, :percent_rank], order: { percent_rank: :asc }
+  end
+
+  def down
+    remove_index :posts, [:topic_id, :percent_rank]
+  end
+end