Refactoring: introduce Slot(s)

We should decouple the ticket from the seat.
Introducing the concept of "slot" (a "seat" in a fixed-seats event) would give us more flexibility with respect to:
- cancelled / expired reservations: we can save the expired or cancelled reservation (including tickets) for reuse
- we can create tickets dinamically as people generate them.
- we can limit the validity of a ticket only to a part of the conference (e.g. single-day access) by linking the ticket to more slots
- etc.

The concept of slot would be general and not strictly related to tickets. It could come in handy also for #111