Bug 24081: Add discount action to boraccount

Bug 24081: Add discount action to boraccount

This patch adds the ability to apply discounts to existing patron
charges from the boraccount page.

Test plan
1/ Carry out some accounting actions that end up with debts on the
patron record. (Add a manual invoice for example)
2/ Note a new 'Apply discount' action button on the unpaid debt
3/ Paid debts do not display the action button
4/ Click the button
5/ Enter an amount into the resulting modal and click confirm
6/ Confirm that the outstanding debt has been reduced by the amount you
specified.
7/ Confirm that a new 'Discount' line appears on the patrons account
8/ Confirm that in the 'details' view of the debt that the discount line
was used against the debt to create the reduction.
9/ Signoff

Signed-off-by: Michal Denar <black23@gmail.com>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>