[B] handling classrooms timers

**Database**
- [ ] Add timer row  (default 6min)

**Controllers & Services**
- [ ] Add timer handling in addClassroom
- [ ] Add timer handling in updateClassroom
- [ ] Include timer value in getClassroom responses 

**Validators**
- [ ] Add timer handling in newClassroomValidation
- [ ] Add timer handling in updateClassroomValidation