Fix #1001 -- Add product bundles (#1041)

* Data model + Editor

* Cart and order management

* Rebase migrations

* Fix typos, add tests on cart handling

* Add tests for checkout and quotas

* Add API endpoints

* Validation of settings

* Front page tax display

* Voucher handling

* Widget foo

* Show correct net pricing

* Front page tests

* reverse charge foo

* Allow to require bundling

* Fix test failure on postgres

src/pretix/base/models/orders.py

@@ -1691,8 +1691,9 @@ class OrderPosition(AbstractPosition):
         # Delete afterwards. Deleting in between might cause deletion of things related to add-ons
         # due to the deletion cascade.
         for cartpos in cp:
-            cartpos.addons.all().delete()
-            cartpos.delete()
+            if cartpos.pk:
+                cartpos.addons.all().delete()
+                cartpos.delete()
         return ops
 
     def __str__(self):
@@ -1789,6 +1790,7 @@ class CartPosition(AbstractPosition):
     includes_tax = models.BooleanField(
         default=True
     )
+    is_bundled = models.BooleanField(default=False)
 
     class Meta:
         verbose_name = _("Cart position")