Bug 22724: Allow only users with correct permissions to 'Write off selected'

Bug 22724: Allow only users with correct permissions to 'Write off selected'

Due to the addition of a 'Write off selected' button in 18.11, users without
writeoff permissions could mistakenly view and action the 'Write off
selected' option for a patron in Accounting > Make a payment (Fines > Pay fines).
The attached patch corrects this error.

Test plan:
1) Make sure that a staff user has '(writeoff) Write off fines and fees'
permission disabled
2) Bring up a patron with an unpaid fee, go to Accounting -> Make a
payment
3) Note that 'Write off selected' button is displayed.
4) Select a fee, click 'Write off selected'. Note that staff member has
access to 'Write off an amount toward selected fines'
5) Apply the patch
6) Repeat steps 1-2, and note that the button is no longer displayed
7) Bonus points, as a staff user with writeoff permissions enabled check
that all four buttons (Pay amount, Pay selected, Write off all, Write
off selected) are displayed

Sponsored-by: Catalyst IT
Signed-off-by: Liz Rea <wizzyrea@gmail.com>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Liz Rea <wizzyrea@gmail.com>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
(cherry picked from commit d36566d6eaa9dd02528bbd1219747ccfd4e6b531)
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>