Fix position of order_by in lessons_on_day query on person model

The order_by wasn't used in the old position because filter_person
uses an union query.

aleksis/apps/chronos/model_extensions.py

@@ -83,7 +83,7 @@ def lesson_periods_as_teacher(self):
 @Person.method
 def lessons_on_day(self, day: date):
     """Get all lessons of this person (either as participant or teacher) on the given day."""
-    return LessonPeriod.objects.order_by("period__period").on_day(day).filter_from_person(self)
+    return LessonPeriod.objects.on_day(day).filter_from_person(self).order_by("period__period")
 
 
 @Person.method