feat: add post_save signal to attach notes to new timeslots

Closes # 214

program/apps.py

+from django.apps import AppConfig
+
+
+class ProgramConfig(AppConfig):
+    name = "program"
+
+    def ready(self):
+        import program.signals  # noqa

program/signals.py

+from django.db.models.signals import post_save
+from django.dispatch import receiver
+from program.models import Note, TimeSlot
+
+
+@receiver(post_save, sender=TimeSlot)
+def attach_note(sender, instance, created, **kwargs):
+    if created:
+        Note.objects.create(timeslot=instance)