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

config/danbooru_default_config.rb

@@ -359,6 +359,9 @@ module Danbooru
     def stripe_publishable_key
     end
 
+    def stripe_webhook_secret
+    end
+
     def twitter_api_key
     end
 

config/initializers/stripe.rb

@@ -1 +1,2 @@
 Stripe.api_key = Danbooru.config.stripe_secret_key
+Stripe.api_version = "2020-08-27"

config/routes.rb

@@ -257,6 +257,9 @@ Rails.application.routes.draw do
   resource :user_upgrade, :only => [:new, :create, :show]
   resources :user_feedbacks, except: [:destroy]
   resources :user_name_change_requests, only: [:new, :create, :show, :index]
+  resources :webhooks do
+    post :receive, on: :collection
+  end
   resources :wiki_pages, id: /.+?(?=\.json|\.xml|\.html)|.+/ do
     put :revert, on: :member
     get :search, on: :collection