Disable the ability to upgrade to Platinum. The Platinum level still
exists, but users can no longer upgrade to it. Sales of Platinum are
being disabled in preparation of increasing the tag limit for Gold
users.
Refactor page limits to a) be explicitly listed in the User class (not
hidden away in the Danbooru config) and b) explicitly depend on the
CurrentUser (not implicitly by way of Danbooru.config.max_numbered_pages).
