Bug 20703: Add ability to void any credit

Bug 20703: Add ability to void any credit

At this time, only payments may be voided. There is no reason to have this limitation. There are situations where a librarian may need to void an accidental writeoff, or perhaps void an automatic credit that was created by Koha. For illustration, this is directly from a partner library:

"For example a lost book refund becomes a credit on account.  Presently the credit may be applied to a fine for a different item charged to patron. We perform a write off to clear the remaining credit, then add the fine back to the account and give the patron a refund for the lost/refunded amount. Our accounting system ask that we keep the Lost funds/refunds separate from all fines."

Test Plan:
1) Create a fine and write it off
2) Note there is no 'void' button for the writeoff
3) Apply this patch
4) Note the buttons now show
5) Test each button on a writeoff

Signed-off-by: Mark Tompsett <mtompset@hotmail.com>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>

Signed-off-by: Nick Clemens <nick@bywatersolutions.com>