Better billing for businesses

A batch of improvements for anyone buying reviews on a company card:

• Your saved business details (company name and tax ID) now show on the purchase page, with a link to update them. Once entered, the checkout itself never displays these again, so the purchase page is now the place to confirm they're right before you buy.
• Want your billing address on the invoice? Tick Add my billing address to the invoice and the checkout will prompt for it. We remember whether you want that prompt; the address itself can't be pre-filled by the checkout yet, so it does need re-entering each time.
• Purchase history now offers a proper invoice PDF download alongside the receipt.