Show holidays and weekends in sick note datepicker too

* To be able to show personal and public holidays in
  datepicker of sick note form, joined the two
  different datepicker functions of sick note and
  application for leave form to one.

* Add missing styling for datepicker in forms for
  holidays and weekends.

* refs #14895
Forms with datepicker: allow to change start date afterwards

* Application for leave form and sick note form:
  allow to change the start date without to
  change the end date when an end date is set.

* refs #14895