New implementation of sales channels (#4111)

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

src/tests/plugins/paypal/test_webhook.py

@@ -49,6 +49,7 @@ def env():
         status=Order.STATUS_PAID,
         datetime=now(), expires=now() + timedelta(days=10),
         total=Decimal('13.37'),
+        sales_channel=o.sales_channels.get(identifier="web"),
     )
     o1.payments.create(
         amount=o1.total,