Create "catch all" categories

[cbellone]

A member of jug Milano, during our presentation, pointed out that it could make sense to have a "catch all" category in the event, and that category should be the one at "full price".
After a little bit of analysis, we really like the original idea, but we would rather implement it as following:

1. add a "behaviour selection" during category creation. Possible behaviours are: ["bounded", "unbounded"].
   • Bounded categories behave exactly like now: they have a maximum number of tickets, a validity period and they can be public or restricted
   • Unbounded categories don't have constraints about minimum/maximum number of tickets (they catch all the "not allocated" tickets) but can't be restricted
2. Add the possibility to define a default unbounded category during event creation/modification. However, an event organizer should be free to decide whether it is worth to insert unbounded/bounded categories or not. Additionally, we should add an help page in order to explain what's the difference.
3. When an expired, bounded category contains orphan tickets, the organizer should be able to allocate them to another bounded category or to "free" them and thus to sell them through an unbounded category
4. At the same time, people should be able to "free" their ticket (please be aware that the refund process, if any, should be managed manually through stripe admin)

This would simplify a lot the implementation of #39, since the waiting queue could be a special unbounded queue.

please feel free to share your thoughts