[FIX] account: allow negative days/months in payment terms

Some companies require their clients to pay them always for the 10th day
of the month (for instance), and if we're the 15th already, then pay the
10th of the next month.

However, with the changes in the payment terms, we can now only ask the
client to pay for the 10th of the next month, which is not satisfying.

Therefore, we now allow negative days/months to meet this demand.

task-id 3127965

closes odoo/odoo#133148

Signed-off-by: Laurent Smet (las) <las@odoo.com>