New implementation of sales channels (#4111)

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

src/tests/api/test_subevents.py

@@ -60,6 +60,7 @@ def order(event, item, taxrule):
             status=Order.STATUS_PENDING, secret="k24fiuwvu8kxz3y1",
             datetime=datetime(2017, 12, 1, 10, 0, 0, tzinfo=timezone.utc),
             expires=datetime(2017, 12, 10, 10, 0, 0, tzinfo=timezone.utc),
+            sales_channel=event.organizer.sales_channels.get(identifier="web"),
             total=23, locale='en'
         )
         o.fees.create(fee_type=OrderFee.FEE_TYPE_PAYMENT, value=Decimal('0.25'), tax_rate=Decimal('19.00'),