Introduce sending of mails when a new orga message is saved

This will send an email with the text and a link to the corresponding AK to the contact mail adress specified for a given event
This implements #116

AKSubmission/models.py

-# Create your models here.
+from django.conf import settings
+from django.core.mail import EmailMessage
+from django.db.models.signals import post_save
+from django.dispatch import receiver
+from django.urls import reverse_lazy
+
+from AKModel.models import AKOrgaMessage
+
+
+@receiver(post_save, sender=AKOrgaMessage)
+def orga_message_saved_handler(sender, instance: AKOrgaMessage, **kwargs):
+    # React to saved (thus new) Orga message by sending an email
+
+    if settings.SEND_MAILS:
+        host = 'https://' + settings.ALLOWED_HOSTS[0] if len(settings.ALLOWED_HOSTS) > 0 else 'http://127.0.0.1:8000'
+        url = f"{host}{reverse_lazy('submit:ak_detail', kwargs={'pk': instance.ak.pk, 'event_slug': instance.ak.event.slug})}"
+
+        mail = EmailMessage(
+            f"[AKPlanning] New message for AK '{instance.ak}' ({instance.ak.event})",
+            f"{instance.text}\n\n{url}",
+            settings.DEFAULT_FROM_EMAIL,
+            [instance.ak.event.contact_email]
+        )
+        mail.send(fail_silently=True)