site(purchase.html): Change name of purchase forms

'foxy_purchase.html' is not the most professional name. Changed in
favor of a more standard name.

The purchase url is still: http://fineuploader.com/purchase.html
This is simply a redirect to purchase_form.html. Discounts are
at purchase_discount_form.html.

The previous foxy_purchase.html and foxy_purchase_discount.html
are still there -- so there aren't broken links -- but redirect to
their respective new pages.