Fix #526 -- Add a webhook system (#1073)

- [x] Data model
- [x] UI
- [x] Fire hooks
- [x] Unit tests
- [x] Display logs
- [x] API to modify hooks
- [x] Documentation
- [x] More hooks!

src/pretix/api/signals.py

@@ -0,0 +1,21 @@
+from datetime import timedelta
+
+from django.dispatch import Signal, receiver
+from django.utils.timezone import now
+
+from pretix.api.models import WebHookCall
+from pretix.base.signals import periodic_task
+
+register_webhook_events = Signal(
+    providing_args=[]
+)
+"""
+This signal is sent out to get all known webhook events. Receivers should return an
+instance of a subclass of pretix.api.webhooks.WebhookEvent or a list of such
+instances.
+"""
+
+
+@receiver(periodic_task)
+def cleanup_webhook_logs(sender, **kwargs):
+    WebHookCall.objects.filter(datetime__lte=now() - timedelta(days=30)).delete()