Bug 16463: Replace discharge link with error message if user has checked out items

Bug 16463: Replace discharge link with error message if user has checked out items

To test:
1) Ensure the useDischarge syspref is enabled
2) Check out an item to a borrower
3) Log in to the OPAC as this borrower
4) Click the 'ask for a discharge' link in the nav
5) Click the 'Ask for a discharge' link
6) Notice you cannot be discharged because you have checkouts
7) Apply the patch, click the 'ask for a discharge' link in the nav
8) Notice the link has been replaced with an appropriate error message
9) Attempt to force the discharge URL:
/cgi-bin/koha/opac-discharge?op=request
10) Notice the message and you cannot be discharged.
11) Confirm that when you check in your item, the discharge link shows
again and works as expected.

Sponsored-by: Catalyst IT
Signed-off-by: Caroline Cyr La Rose <caroline.cyr-la-rose@inlibro.com>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>