Fix public_speakers

The `persons` variable is a list, not a set, so we should use `append`
instead of `add`.

Fixes #604

src/core/models/events.py

@@ -175,7 +175,7 @@ class Event(TaggedItemMixin, BackendMixin, ActivityLogMixin, models.Model):
         persons = [participant.participant for participant in self.participants.filter(is_public=True, role=EventParticipant.Role.SPEAKER)]
 
         if len(persons) == 0 and self.kind == Event.Kind.SELF_ORGANIZED and self.owner:
-            persons.add(self.owner)
+            persons.append(self.owner)
 
         return persons