PERF: Add index_reply_id_on_post_replies.

Speeds up the reference posts query in `Email::Sender#send`.

db/migrate/20190410055459_add_index_reply_id_on_post_replies.rb

@@ -0,0 +1,5 @@
+class AddIndexReplyIdOnPostReplies < ActiveRecord::Migration[5.2]
+  def change
+    add_index :post_replies, :reply_id
+  end
+end

lib/email/sender.rb

@@ -110,7 +110,8 @@ module Email
           "<topic/#{topic_id}/#{post_id}@#{host}>"
 
         referenced_posts = Post.includes(:incoming_email)
-          .where(id: PostReply.where(reply_id: post_id).select(:post_id))
+          .joins("INNER JOIN post_replies ON post_replies.post_id = posts.id ")
+          .where("post_replies.reply_id = ?", post_id)
           .order(id: :desc)
 
         referenced_post_message_ids = referenced_posts.map do |referenced_post|