Bug 29231: Add missing Flatpickr to inventory page

Bug 29231: Add missing Flatpickr to inventory page

This patch corrects a stray instance of the "datepicker" class in the
inventory template and replaces it with "flatpickr" to trigger a
Flatpickr.

To test, apply the patch and go to Tools -> Inventory. Check that the
"Last inventory date" field is a Flatpickr widget.

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>