Online Student Scheduling: Enroll (Hiberante 6.6 Fix)

- fixed a Hibernate error: persistent instance references an unsaved transient instance of CourseDemand
  - thrown when dropping a course demand that was wait-listed in the past
  - likely caused by Student.resetWaitLists(..) touching previuous WaitList records that refer the course demand that was just deleted

JavaSource/org/unitime/timetable/onlinesectioning/updates/EnrollStudent.java

@@ -784,6 +784,7 @@ else if (!courseId.equals(ca.getCourseId())) {
 				helper.getHibSession().merge(student);
 
 				if (wlMode == WaitListMode.WaitList) {
+					helper.getHibSession().flush();
 					student.resetWaitLists(
 							WaitList.WaitListType.SCHEDULING_ASSISTANT,
 							helper.getUser().getExternalId(),