user upgrades: add upgrade code system.

Add a system for upgrading accounts using upgrade codes. Users purchase
an upgrade code off-site then redeem it on-site to upgrade their account
to Gold. Upgrade codes are randomly pre-generated and are one time use
only. Codes have enough randomness that guessing a code is infeasible.

db/migrate/20220525214746_create_upgrade_codes.rb

@@ -0,0 +1,22 @@
+class CreateUpgradeCodes < ActiveRecord::Migration[7.0]
+  def change
+    create_table :upgrade_codes do |t|
+      t.timestamps
+      t.string :code, null: false
+      t.integer :status, null: false
+      t.integer :creator_id, null: false
+      t.integer :redeemer_id
+      t.integer :user_upgrade_id
+
+      t.index :code, unique: true
+      t.index :status
+      t.index :creator_id
+      t.index :redeemer_id, where: "redeemer_id IS NOT NULL"
+      t.index :user_upgrade_id, where: "user_upgrade_id IS NOT NULL"
+
+      t.foreign_key :users, column: :creator_id
+      t.foreign_key :users, column: :redeemer_id
+      t.foreign_key :user_upgrades, column: :user_upgrade_id
+    end
+  end
+end