Simple double opt-in web app for Postmark mailing list

If you’re using an email service like Postmark, you’ll have to handle your own list management except unsubscribes and suppression (e.g. from hard bounces). To do this, a little web app will be needed if you want to do things like double opt-in (good idea). You’ll likely then want more features like profile management but the goal of this howto is getting a simple and hopefully secure webapp in place for /subscribe and /confirm (you can get unsubscribes/suppressions from Postmark via web-hooks or an API call).