Add holidays routes to backend and frontend; implement holiday associations and update UI components for admin holidays management

backend/src/models/index.js

@@ -9,6 +9,7 @@ const AuthInfo = require('./AuthInfo');
 const State = require('./State');
 const WeeklyWorktime = require('./WeeklyWorktime');
 const Holiday = require('./Holiday');
+const HolidayState = require('./HolidayState');
 const Vacation = require('./Vacation');
 const Sick = require('./Sick');
 const SickType = require('./SickType');
@@ -22,6 +23,7 @@ module.exports = {
   State,
   WeeklyWorktime,
   Holiday,
+  HolidayState,
   Vacation,
   Sick,
   SickType,