Ticket allocation

[marksteward]

There should be a specified number of each ticket available (although we only need to care about pre-sales tickets for the initial release, so we can hard-code some of this.)
When a bank transfer or gocardless payment is started, a ticket should be allocated. This allocation should expire after a configured amount of time.

[russss]

I started on models for this here: https://github.com/emfcamp/Website/blob/master/models/ticket.py

[marksteward]

Fixed with accidental merge of prepay branch. Expiry moved to #13.