user upgrades: add UserUpgrade model.

Add a model to store the status of user upgrades.

* Store the upgrade purchaser and the upgrade receiver (these are
  different for a gifted upgrade, the same for a self upgrade).
* Store the upgrade type: gold, platinum, or gold-to-platinum upgrades.
* Store the upgrade status:
** pending: User is still on the Stripe checkout page, no payment
   received yet.
** processing: User has completed checkout, but the checkout status in
   Stripe is still 'unpaid'.
** complete: We've received notification from Stripe that the payment
   has gone through and the user has been upgraded.
* Store the Stripe checkout ID, to cross-reference the upgrade record on
  Danbooru with the checkout record on Stripe.

This is the upgrade flow:

* When the user clicks the upgrade button on the upgrade page, we call
  POST /user_upgrades and create a pending UserUpgrade.
* We redirect the user to the checkout page on Stripe.
* When the user completes checkout on Stripe, Stripe sends us a webhook
  notification at POST /webhooks/receive.
* When we receive the webhook, we check the payment status, and if it's
  paid we mark the UserUpgrade as complete and upgrade the user.
* After Stripe sees that we have successfully processed the webhook,
  they redirect the user to the /user_upgrades/:id page, where we show
  the user their upgrade receipt.

test/unit/user_upgrade_test.rb

@@ -3,37 +3,45 @@ 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!
+      context "for a self upgrade" do
+        context "to Gold" do
+          setup do
+            @user_upgrade = create(:self_gold_upgrade)
           end
 
-          assert_equal(true, @user.reload.is_builder?)
+          should "update the user's level if the payment status is paid" do
+            @user_upgrade.process_upgrade!("paid")
+
+            assert_equal(User::Levels::GOLD, @user_upgrade.recipient.level)
+            assert_equal("complete", @user_upgrade.status)
+          end
+
+          should "not update the user's level if the payment is unpaid" do
+            @user_upgrade.process_upgrade!("unpaid")
+
+            assert_equal(User::Levels::MEMBER, @user_upgrade.recipient.level)
+            assert_equal("processing", @user_upgrade.status)
+          end
+
+          should "not update the user's level if the upgrade status is complete" do
+            @user_upgrade.update!(status: "complete")
+            @user_upgrade.process_upgrade!("paid")
+
+            assert_equal(User::Levels::MEMBER, @user_upgrade.recipient.level)
+            assert_equal("complete", @user_upgrade.status)
+          end
+
+          should "log an account upgrade modaction" do
+            assert_difference("ModAction.user_account_upgrade.count") do
+              @user_upgrade.process_upgrade!("paid")
+            end
+          end
+
+          should "send the recipient a dmail" do
+            assert_difference("@user_upgrade.recipient.dmails.received.count") do
+              @user_upgrade.process_upgrade!("paid")
+            end
+          end
         end
       end
     end