Bug 35341: Improve processing hard due date and auto renewal date

Bug 35341: Improve processing hard due date and auto renewal date

After quite a struggle, came up with this approach using
flatpickr's parseDate and setDate. Seems to be the best.
The dateformat variable is set in calendar.inc.

Test plan:
Test edit, save, clear rules, focus on both dates.
Toggle all I18N preference date formats.

Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Bug 35341: Do not reinstantiate flatpickr

We do not need to reinstantiate flatpickr, we need to retrieve the
existing instance.

Test plan:
See previous patch.

Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
12-01-2024 Squashed both patches. Removed the unused hidden inputs for
hardduedatecompare and added a data attribute for the duedate.
The check with is_valid_date seems no longer needed here. Normally
date is empty or valid. A js warning from parseDate should be
exceptional. Made code for both dates a bit more consistent.

Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Katrin Fischer <katrin.fischer@bsz-bw.de>