Bug 28963: Use Flatpickr on calendar page

Bug 28963: Use Flatpickr on calendar page

This patch replaces the use of jQueryUI's datepicker widget on the
Calendar page.

In order to ease customization of the static calendar which shows the
current calendar, I've converted the calendar.css file to SCSS. This
allows us to define new SCSS variables to pass to the Flatpickr SCSS.

Also changed: Removed some unecessary comments in the template.

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

- Go to Tools -> Calendar.
- Confirm that entering holidays works correctly for all types: Single,
  weekly, yearly, ranges, repeated ranges.
- Confirm that holidays are deleted as expected.
- Confirm that the colors of each type of holiday is correct.

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>