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/helpers/user_upgrades_helper.rb

@@ -1,24 +1,4 @@
 module UserUpgradesHelper
-  def stripe_button(desc, cost, user)
-    html = %{
-      <form action="#{user_upgrade_path}" method="POST" class="stripe">
-        <input type="hidden" name="authenticity_token" value="#{form_authenticity_token}">
-        #{hidden_field_tag(:desc, desc)}
-        #{hidden_field_tag(:user_id, user.id)}
-        <script
-          src="https://checkout.stripe.com/checkout.js" class="stripe-button"
-          data-key="#{Danbooru.config.stripe_publishable_key}"
-          data-name="#{Danbooru.config.canonical_app_name}"
-          data-description="#{desc}"
-          data-label="#{desc}"
-          data-amount="#{cost}">
-        </script>
-      </form>
-    }
-
-    raw(html)
-  end
-
   def cents_to_usd(cents)
     number_to_currency(cents / 100, precision: 0)
   end