Added instructor availability model and admin

cntmanage/flightslot/models/weekpref.py

@@ -20,7 +20,7 @@ class WeekPreference(models.Model):
         Student,
         null=False,
         db_index=True,
-        on_delete=models.DO_NOTHING,
+        on_delete=models.CASCADE,
         verbose_name="Student Selection"
     )