Bug 29129: Update DisplayClearScreenButton to allow for a choice between issueslip...

Bug 29129: Update DisplayClearScreenButton to allow for a choice between issueslip and issueqslip

To Test:
1. Apply patch, updatedatabase, and restart_all
2. A small change the global scss file means you should regenerate the CSS as well. ( https://wiki.koha-community.org/wiki/Working_with_SCSS_in_the_OPAC_and_staff_interface )
3. Set DisplayClearScreenButton to "don't show"
4. Go to the patron checkout screen and see that no button should show to clear the screen and print
5. Set DisplayClearScreenButton to 'ISSUESLIP' and make sure the button now appears and the ISSUESLIP prints
6. Set DisplayClearScreenButton to 'ISSUEQSLIP' and make sure the button now appears and the ISSUEQSLIP prints

Signed-off-by: Andrew Fuerste-Henry <andrew@bywatersolutions.com>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>