Bug 30927: Improve formatting or iCal files for checkout due dates

Bug 30927: Improve formatting or iCal files for checkout due dates

We got some feedback from a patron regarding the default notifications
on the iCal export for the patron checkout summary in the OPAC.
The notifications send 1 minute before the event (the due date), which
is midnight.

Test Plan:
0) Make sure your time zone in Koha is set correctly in koha-conf.xml
1) Apply this patch
2) Restart all the things!
3) Make a checkout due at 23:59
4) Make a checkout due at 11:00
5) Download the iCal file from the OPAC
6) Import this file into Calendar
7) Note the first checkout is an all day event
8) Note the second checkout is an "instantaneous" event at the date and
   time the item is due

Signed-off-by: Owen Leonard <oleonard@myacpl.org>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>