PERF: Add index `id DESC, baked_version` ON `posts`.

A scheduled job runs `Post.rebake_old` with a limit of 80 which does a
look up for the latest posts that have not been baked to the latest
version. Before this commit, the query would run using the primary key's
index and does a reverse scan. However the query performance quicky
becomes bad as more and more of the latest posts have been baked to the
latest version.

db/migrate/20190408072550_add_index_id_baked_version_on_posts.rb

@@ -0,0 +1,7 @@
+class AddIndexIdBakedVersionOnPosts < ActiveRecord::Migration[5.2]
+  def change
+    add_index :posts, [:id, :baked_version],
+      order: { id: :desc },
+      where: "(deleted_at IS NULL)"
+  end
+end