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

test/unit/user_upgrade_test.rb

@@ -0,0 +1,41 @@
+require 'test_helper'
+
+class UserUpgradeTest < ActiveSupport::TestCase
+  context "UserUpgrade:" do
+    context "the #process_upgrade! method" do
+      setup do
+        @user = create(:user)
+        @user_upgrade = UserUpgrade.new(recipient: @user, purchaser: @user, level: User::Levels::GOLD)
+      end
+
+      should "update the user's level" do
+        @user_upgrade.process_upgrade!
+        assert_equal(User::Levels::GOLD, @user.reload.level)
+      end
+
+      should "log an account upgrade modaction" do
+        assert_difference("ModAction.user_account_upgrade.count") do
+          @user_upgrade.process_upgrade!
+        end
+      end
+
+      should "send the user a dmail" do
+        assert_difference("@user.dmails.received.count") do
+          @user_upgrade.process_upgrade!
+        end
+      end
+
+      context "for an upgrade for a user above Platinum level" do
+        should "not demote the user" do
+          @user.update!(level: User::Levels::BUILDER)
+
+          assert_raise(User::PrivilegeError) do
+            @user_upgrade.process_upgrade!
+          end
+
+          assert_equal(true, @user.reload.is_builder?)
+        end
+      end
+    end
+  end
+end