Handle sell-out conditions

[chrisjrn]

• Make it clear to people before registering that conference has sold out unless they have a voucher code
• Make it possible to enter your voucher code even in sell-out condition (entering a voucher code may enable a new ticket type, for instance)
• Make sure that guided registration fails if there are no ticket-type products available to the user.

[chrisjrn]

Fixed