Web-app: diplay create button even if there are no existing groups
Web-app: fail on invalid department id
Web-app: warn about empty departments
Web-app: Display department name instead of id
Web-app: fixed undefined variable
Web-app: more error handling on viewStudentAttendance page
Web-app: added missing check
Web-app: Fixed checkAttendance page footer
Web-app: validate group when checking student attendance
Web-app: more validation for "make excuse" pages