Clean up of application service: use more generic dao methods

* refs #18516
Adjust API to send more information about holidays

* API sends now information about day length of public
  and personal holidays.

* Adjust overview calendar and datepicker to use this information
  instead of hard coded rules for half work days.

* refs #18516
Enable public holiday description on hover in calendar

* refs #14787
Enable navigation from overview calendar to applications for leave

* It's possible now to click on a date marked as personal holiday in
  overview calendar to navigate to the correspondent application for
  leave.

* refs #17461
Enable selection of today's date in overview calendar

* refs #18523