PERF: Add index_for_rebake_old to posts.

The index becomes smaller over time and is much faster.

Follow up to 4791d992dc.

db/migrate/20190409054736_add_index_for_rebake_old_on_posts.rb

@@ -0,0 +1,22 @@
+class AddIndexForRebakeOldOnPosts < ActiveRecord::Migration[5.2]
+  disable_ddl_transaction!
+
+  def up
+    remove_index :posts, name: :index_posts_on_id_and_baked_version
+
+    add_index :posts, :id,
+      order: { id: :desc },
+      where: "(baked_version IS NULL OR baked_version < 2) AND deleted_at IS NULL",
+      name: :index_for_rebake_old,
+      algorithm: :concurrently
+  end
+
+  def down
+    remove_index :posts, name: :index_for_rebake_old
+
+    add_index :posts, [:id, :baked_version],
+      order: { id: :desc },
+      where: "(deleted_at IS NULL)",
+      algorithm: :concurrently
+  end
+end