Implement notifications for admin users (#700)

* First stab at notification settings

* Add "global" setting for notification levels

* Trigger notification task

* Get users with permission for event

* Actually send notification emails

* More notifications

* Allow to turn off notifications

* Link in email to pause all notifications

* Add NotificationType to wordlist

* Add notification tests

* Add documentation

* Rebase fixes

src/pretix/base/models/base.py

@@ -47,6 +47,8 @@ class LoggingMixin:
         """
         from .log import LogEntry
         from .event import Event
+        from ..notifications import get_all_notification_types
+        from ..services.notifications import notify
 
         event = None
         if isinstance(self, Event):
@@ -60,6 +62,9 @@ class LoggingMixin:
             logentry.data = json.dumps(data, cls=CustomJSONEncoder)
         logentry.save()
 
+        if action in get_all_notification_types():
+            notify.apply_async(args=(logentry.pk,))
+
 
 class LoggedModel(models.Model, LoggingMixin):