Bug 30183: Fetch flatPickr instance prior to custom search definition

Bug 30183: Fetch flatPickr instance prior to custom search definition

This patch updates teh search customisations in ill-list-table to
properly fetch the existing flatpickr instances for date searches.

To test:
1. Have at least one ILL request on the ILL requests page
2. Open the broser inspector
3. Choose a date on the left hand form
=> FAIL: Errors in the console, search doesn't work
4. Apply this patch
5. Reload
6. Repeat 2-3
=> SUCCESS: No more errors, filtering works!
7. Sign off :-D

Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Fridolin Somers <fridolin.somers@biblibre.com>