Add sales channels (#1103)

- [x] Data model
- [x] Enforce constraint
- [x] Filter order list
- [x] Set channel on created order
- [x] Products API
- [x] Order API
- [x] Tests
- [x] Filter reports
- [x] Resellers
- [ ] deploy plugins
  - [ ] posbackend
  - [ ] resellers
  - [ ] reports
- [x] Ticketlayouts
- [x] Support in pretixPOS

src/pretix/base/models/orders.py

@@ -94,6 +94,8 @@ class Order(LockModel, LoggedModel):
     :type require_approval: bool
     :param meta_info: Additional meta information on the order, JSON-encoded.
     :type meta_info: str
+    :param sales_channel: Identifier of the sales channel this order was created through.
+    :type sales_channel: str
     """
 
     STATUS_PENDING = "n"
@@ -174,6 +176,7 @@ class Order(LockModel, LoggedModel):
     require_approval = models.BooleanField(
         default=False
     )
+    sales_channel = models.CharField(max_length=190, default="web")
 
     class Meta:
         verbose_name = _("Order")