Introduce sending of mails

Introduce sending of mails when a new orga message is saved, and when a new AKSlot is added even though the plan is already visible.

Introduce necessary basic settings for that feature (including the usage of a debug email backend for dev setups).

Sends an email with the text and a link to the corresponding AK to the contact mail address specified for a given event (implements #116 (closed)).

Sends an email with information about the slot and a link to the corresponding AK to the contact mail address specified for a given event, but only if the plan feature is used, the plan is already visible and the slot was not directly scheduled (implements #117 (closed)).

The mails are always in English, since there is no "default language" for an event and there is also no client language that could be used by the translation feature when sending mails.

AKSubmission/models.py

+from django.apps import apps
 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
+from AKModel.models import AKOrgaMessage, AKSlot
 
 
 @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
+def orga_message_saved_handler(sender, instance: AKOrgaMessage, created, **kwargs):
+    # React to newly created Orga message by sending an email
 
-    if settings.SEND_MAILS:
+    if created and 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})}"
 
@@ -22,3 +23,20 @@ def orga_message_saved_handler(sender, instance: AKOrgaMessage, **kwargs):
             [instance.ak.event.contact_email]
         )
         mail.send(fail_silently=True)
+
+
+@receiver(post_save, sender=AKSlot)
+def slot_created_handler(sender, instance: AKSlot, created, **kwargs):
+    # React to slots that are created after the plan was already published by sending an email
+
+    if created and settings.SEND_MAILS and apps.is_installed("AKPlan") and not instance.event.plan_hidden and instance.room is None and instance.start is None:
+        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 slot for AK '{instance.ak}' ({instance.ak.event}) added",
+            f"New slot requested.\n\nAK: {instance.ak}\nDuration: {instance.duration}\n\n{url}",
+            settings.DEFAULT_FROM_EMAIL,
+            [instance.ak.event.contact_email]
+        )
+        mail.send(fail_silently=True)