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.

app/policies/upgrade_code_policy.rb

@@ -0,0 +1,15 @@
+# frozen_string_literal: true
+
+class UpgradeCodePolicy < ApplicationPolicy
+  def index?
+    user.is_owner?
+  end
+
+  def redeem?
+    true
+  end
+
+  def upgrade?
+    unbanned?
+  end
+end