Bug 15823: Can still access patron discharge slip without having the syspref on

Bug 15823: Can still access patron discharge slip without having the syspref on

EDIT: Fix for OPAC side
EDIT: Comment 10
EDIT: Merge conflicts

To test:
1) Ensure syspref useDischarge is disabled
2) Go to /cgi-bin/koha/members/discharge.pl?borrowernumber=X&discharge=1
3) Validate that you are still able to generate a discharge slip for this patron
4) Apply patch and refresh page
5) Confirm that you are redirected to the circulation.pl page for the user and that an error message is there.
OPAC SIDE
6) Go to the OPAC
7) Go to /cgi-bin/koha/opac-discharge.pl
8) Confirm you get a message saying discharges are disabled
9) Go to /cgi-bin/koha/opac-discharge.pl?op=request
10) Confirm you see same message

Sponsored-by: Catalyst IT
Followed test plan, works as expected (both staff client and OPAC).
Re-tested, works OK.
Signed-off-by: Marc Véron <veron@veron.ch>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Brendan Gallagher <brendan@bywatersolutions.com>