Add order-level telephone field to core (#1872)

Co-authored-by: Martin Gross <gross@rami.io>

src/pretix/api/serializers/event.py

@@ -601,6 +601,9 @@ class EventSettingsSerializer(serializers.Serializer):
         'attendee_data_explanation_text',
         'confirm_texts',
         'order_email_asked_twice',
+        'order_phone_asked',
+        'order_phone_required',
+        'checkout_phone_helptext',
         'payment_term_mode',
         'payment_term_days',
         'payment_term_weekdays',