feat: Holiday office multiselect replaced with multiselect

src/app/organization/holidays/create-holiday/todo-flat-item.class.ts

@@ -0,0 +1,10 @@
+import { Injectable } from '@angular/core';
+
+@Injectable({
+    providedIn: 'root'
+})
+export class TodoItemFlatNode {
+    item: string;
+    level: number;
+    expandable: boolean;
+  }