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/user_upgrades_controller.rb

@@ -1,11 +1,12 @@
 class UserUpgradesController < ApplicationController
   helper_method :user
-  skip_before_action :verify_authenticity_token, only: [:create]
+  respond_to :js, :html
 
   def create
-    if params[:stripeToken]
-      create_stripe
-    end
+    @user_upgrade = UserUpgrade.new(recipient: user, purchaser: CurrentUser.user, level: params[:level].to_i)
+    @checkout = @user_upgrade.create_checkout
+
+    respond_with(@user_upgrade)
   end
 
   def new
@@ -22,38 +23,4 @@ class UserUpgradesController < ApplicationController
       CurrentUser.user
     end
   end
-
-  private
-
-  def create_stripe
-    @user = user
-
-    if params[:desc] == "Upgrade to Gold"
-      level = User::Levels::GOLD
-      cost = UserUpgrade.gold_price
-    elsif params[:desc] == "Upgrade to Platinum"
-      level = User::Levels::PLATINUM
-      cost = UserUpgrade.platinum_price
-    elsif params[:desc] == "Upgrade Gold to Platinum" && @user.level == User::Levels::GOLD
-      level = User::Levels::PLATINUM
-      cost = UserUpgrade.upgrade_price
-    else
-      raise "Invalid desc"
-    end
-
-    begin
-      charge = Stripe::Charge.create(amount: cost, currency: "usd", source: params[:stripeToken], description: params[:desc])
-      @user.promote_to!(level, User.system, is_upgrade: true)
-      flash[:success] = true
-    rescue Stripe::StripeError => e
-      DanbooruLogger.log(e)
-      flash[:error] = e.message
-    end
-
-    if @user == CurrentUser.user
-      redirect_to user_upgrade_path
-    else
-      redirect_to user_upgrade_path(user_id: params[:user_id])
-    end
-  end
 end