Bug 28663: Prevent discounts against void lines

Bug 28663: Prevent discounts against void lines

With the introduction of double entry accounting for VOID actions, we
need to add an additional filter to the 'Apply discount' button appearance

Test plan
1/ Add a debt
2/ Pay the debt
3/ Void the payment
4/ Confirm that with the patch applied the 'Apply discount' button does
not appear on the 'Void' accountline.

Signed-off-by: Michal Denar <black23@gmail.com>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Fridolin Somers <fridolin.somers@biblibre.com>