Bug 19617: Allow 'writeoff of selected'

Bug 19617: Allow 'writeoff of selected'

This patch adds a writeoff equivilent to 'Pay selected'

Test Plan:
1) Apply this patch
2) Find a patron with fines
3) On the "Pay fines" tab, select one or more fines and use the
   "Write off selected" button.
4) Note the fine amount you used was written off for those fines.

Signed-off-by: Kyle M Hall <kyle@bywatetsolutions.com>
Signed-off-by: Martha Fuerst <mfuerst@hmcpl.org>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>