Bug 33345: On-site checkout checkbox does not work since issue date using flatpickr

Bug 33345: On-site checkout checkbox does not work since issue date using flatpickr

Since Bug 30718, date due uses flatpickr.
The JavaScript code setting due date via on-site checkout checkbox needs to be adapted.
Sort of like Bug 30717.

This patch renames the variable sent to template
'today_due_date_and_time' to avoid confusion with variable 'todaysdate'
used in other templates.

Test plan :
1) Enable system preference 'OnSiteCheckouts'
2) Go to a patron circulation page /cgi-bin/koha/circ/circulation.pl
3) Click on settings icon inside barcode input
=> Check due date input is empty
4) Click on 'On-site checkout'
=> Check due date input is filled with today date at 23:59
5) Fill barcode input and do check-out
6) Check due date is correct

Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>