Bug 30718: Use flatpickr's altInput

Bug 30718: Use flatpickr's altInput

The idea rely on the KohaDates TT plugin for the date formatting. We
should not have any output_pref calls in pl or pm (there are some
exceptions, for ILSDI for instance).

Also flatpickr will deal with the places where dates are inputed. We
will pass the raw SQL value (what we call 'iso' in Koha::DateUtils), and
the controller will receive the same value, no need to additional
conversion.
Note that DBIC has the capability to auto-deflate DateTime objects,
which makes things way easier. We can either pass the value we receive
from the controller, or pass a DT object to our methods.

Signed-off-by: Victor Grousset/tuxayo <victor@tuxayo.net>
Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>