update can_cancel to respect reservation state

shiftregister/app/models.py

@@ -120,6 +120,8 @@ class ShiftRegistration(models.Model):
     )
 
     def can_cancel(self):
+        if self.state != self.REGISTERED:
+            return false
         return self.shift.start_at > (
             timezone.now()
             + global_preferences_registry.manager()["helper__min_cancel_time"]