personal workdays, refactoring of calendar service

* getWorkDays method uses now WorkingTime object for calculation
* adjusted classes that uses this method of the calendar service
* adjusted javascript and controller for frontend calculation of work
  days

refs #10962
personal workdays, added hidden validForm attribute

* now a person may have multiple WorkingTime objects
* calculation for working days considers the validities

refs #10962
datepicker for vacation overview

* integrated in overview page
* implemented Controller method that returns json with vacation days
* implemented datepicker instance

refs #8755
datepicker for overview, now showing 5 months

* included date.js library
* numberOfMonths set to 5
* adjusted getPublicHolidays and getHolidays methods

refs #8755
overview datepicker, changed color for today's date

refs #8755
datepicker overview, highlight only allowed apps

refs #8755