Bug 29241: Allow display of past date for .futuredate

Bug 29241: Allow display of past date for .futuredate

When a past date is set for a flatpickr instance with the .futuredate
class, only dates in the future are available AND the input is blanked.
It does not display the date in the past.

For instance if a hold is expired (expirationdate in the past), the date input will be blanked.

We can use the flatpickr's allowInvalidPreload option to allow date in the past to be displayed.

Test plan:
Place a hold
Edit its expirationdate and set a date in the past (yesterday is fine)
Go to /reserve/request.pl?biblionumbers=42
=> With the patch you see the date, and the widget let you chose anoter date,
in the future
=> Without this patch the expiration date is not displayed

Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>