Define a discountPolicy for each category

[yankedev]

It could be useful to apply a special discount for reservations with many tickets (in the same transaction).
For example apply 10% discount if you buy 5 or more tickets.
Another nice policy could be to do:

• get 6 tickets, pay 5
• get 11 tickets, pay 9
• get 15 tickets, pay 12

The discount policy should be applied before any VOUCHER discount and must be visualized in the confirmation page and in the final invoice

[cbellone]

will be implemented as defined in #869