add designatedTrainees ManytoManyField in WorkerGroup, remove the QualificationGroup. Add ExceptionRequest Model. Remove ExceptionGroup model and permanentdesignation model. Therefore, now, service has qualified trainee, instane has exceptional trainee and workergroup has designated trainees. We define Qualification, exception and designation in the following way. Qualificatioin means you are qualified to serve in a certain service(skills, TA assigned, etc). Excetion means you are not be able to serve in a certain instance(For example, YP can't serve Saturday Dinner, etc). Designation is a a trainee is a designated to serve in a particular workgroup.
forget the remove the PermanentDesignation in the preivous commit ,so remove it