Bug 28945: Use Flatpickr on administration pages

Bug 28945: Use Flatpickr on administration pages

This patch updates administration templates to use Flatpickr instead of
jQueryUI datepickers.

To test, apply the patch and test date fields on the following pages:

- Administration -> Budgets ->
  - New budget: Linked "Start date" and "End  date" fields.
  - Duplicate budget: Linked "Start date" and "End  date" fields.
- Acquisitions -> Vendor -> New contract: Linked "Start date" and "End
  date" fields.
- Administration -> Circulation and fine rules: "Hard due date" and "No
  automatic renewal after" fields.

Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>