Send purchasers an email when they have made a purchase

[chrislo]

At the moment we redirect to a purchase show page after Stripe successfully processes a payment. It would be good to send the customer an email too because at the moment there's no obvious way for them to go pack to the purchase page later (unless they save the URL somewhere).

To do this I think we'll need to listen to the stripe webhook for a successful purchase and potentially mark the purchase record as complete (so we don't send emails multiple times).