user upgrades: upgrade to new Stripe checkout system.

This upgrades from the legacy version of Stripe's checkout system to the
new version:

> The legacy version of Checkout presented customers with a modal dialog
> that collected card information, and returned a token or a source to
> your website. In contrast, the new version of Checkout is a smart
> payment page hosted by Stripe that creates payments or subscriptions. It
> supports Apple Pay, Dynamic 3D Secure, and many other features.

Basic overview of the new system:

* We send the user to a checkout page on Stripe.
* Stripe collects payment and sends us a webhook notification when the
  order is complete.
* We receive the webhook notification and upgrade the user.

Docs:

* https://stripe.com/docs/payments/checkout
* https://stripe.com/docs/payments/checkout/migration#client-products
* https://stripe.com/docs/payments/handling-payment-events
* https://stripe.com/docs/payments/checkout/fulfill-orders

app/controllers/webhooks_controller.rb

@@ -0,0 +1,13 @@
+class WebhooksController < ApplicationController
+  skip_forgery_protection only: :receive
+  rescue_with Stripe::SignatureVerificationError, status: 400
+
+  def receive
+    if params[:source] == "stripe"
+      UserUpgrade.receive_webhook(request)
+      head 200
+    else
+      head 400
+    end
+  end
+end