Customer wants to require a down payment of 30 % for bookings made
one week or more before the actual date, and to make the full payment
otherwise.
This would require yet another relation to keep these values. Fuck it;
i added them to the function, as they are very unlikely to change.
That forced me to change the test for draft_payment to use relative
dates, otherwise there is no way i can have stable results in the
future.
