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/base/models/log.py

@@ -63,6 +63,16 @@ class LogEntry(models.Model):
                 return response
         return self.action_type
 
+    @cached_property
+    def organizer(self):
+        if self.event:
+            return self.event.organizer
+        elif hasattr(self.content_object, 'event'):
+            return self.content_object.event.organizer
+        elif hasattr(self.content_object, 'organizer'):
+            return self.content_object.organizer
+        return None
+
     @cached_property
     def display_object(self):
         from . import Order, Voucher, Quota, Item, ItemCategory, Question, Event, TaxRule, SubEvent