Fixed #107 and fixed #125 -- Periodic cleanup tasks

src/pretix/base/signals.py

@@ -95,3 +95,13 @@ don't know how to turn it into human-readable text.
 logentry_display = EventPluginSignal(
     providing_args=["logentry"]
 )
+
+
+"""
+This is a regular django signal (no pretix event signal) that we send out every
+time the periodic task cronjob runs. This interval is not sharply defined, it can
+be everything between a minute and a day. The actions you perform should be
+idempotent, i.e. it should not make a difference if this is send out more often
+than expected.
+"""
+periodic_task = django.dispatch.Signal()