Fix sendmail migration activating plugin on events (#6656)

src/pretix/base/services/notifications.py

@@ -172,10 +172,11 @@ def send_notification_mail(notification: Notification, user: User):
     guid = uuid.uuid4()
     settings_holder = notification.event or notification.organizer
     prefix = settings_holder.settings.mail_prefix
-    if not prefix and notification.event:
-        prefix = notification.event.slug.upper()
-    elif notification.organizer:
-        prefix = notification.organizer.name
+    if not prefix:
+        if notification.event:
+            prefix = notification.event.slug.upper()
+        elif notification.organizer:
+            prefix = notification.organizer.name
     m = OutgoingMail.objects.create(
         guid=guid,
         user=user,

src/pretix/plugins/sendmail/migrations/0013_activate_plugin_for_events_with_scheduled_mails.py

@@ -19,7 +19,9 @@ def activate_plugin(apps, schema_editor):
     )
 
     for event in events:
-        event.enable_plugin('pretix.plugins.sendmail')
+        plugins_active = event.plugins.split(',')
+        plugins_active.append('pretix.plugins.sendmail')
+        event.plugins = ','.join(plugins_active)
         event.save(update_fields=['plugins'])