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.

db/migrate/20201224101208_create_user_upgrades.rb

@@ -0,0 +1,20 @@
+class CreateUserUpgrades < ActiveRecord::Migration[6.1]
+  def change
+    create_table :user_upgrades do |t|
+      t.timestamps
+
+      t.references :recipient, index: true, null: false
+      t.references :purchaser, index: true, null: false
+      t.integer :upgrade_type, index: true, null: false
+      t.integer :status, index: true, null: false
+      t.string :stripe_id, index: true, null: true
+    end
+
+    # Reserve ID space for backfilling old upgrades.
+    reversible do |dir|
+      dir.up do
+        execute "SELECT setval('user_upgrades_id_seq', 25000, false)"
+      end
+    end
+  end
+end