Bug 28982: Use Flatpickr on onboarding pages

Bug 28982: Use Flatpickr on onboarding pages

This patch replaces the use of jQueryUI's datepicker in the onboarding
templates. The addition of calendar.inc to the template requires that
some additions be made to the installer's header files, adding the i18n
JavaScript assets.

jQueryUI-specific CSS is removed from installer.scss, and inclusion of
Flatpickr's SCSS is added.

To test, apply the patch and rebuild the staff client SCSS
(https://wiki.koha-community.org/wiki/Working_with_SCSS_in_the_OPAC_and_staff_client).

- Start the installation process.
- Continue through the installation process until you reach the stage in
  onboarding where you are creating a patron category.
- The "Enrollment period -> Until" field should be a Flatpickr calendar
  widget. The widget should look and work correctly.

Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>