Refactor cancelling positions and orders in the data model (#1088)

- [x] Data model
- [x] display in order view in backend
- [x] review all usages of OrderPositions.objects
- [x] review all usages of order.positions
- [x] review all other model usages
- [x] review plugins
- [x] plugins backwards-compatible API?
- [x] decide on way forward for REST API
- [x] need to cancel fees
- [x] tests
- [ ] plugins
  - [ ] gdpr
  - [ ] reports
- [x] docs

src/pretix/control/forms/orders.py

@@ -367,8 +367,7 @@ class OrderRefundForm(forms.Form):
         required=False,
         widget=forms.RadioSelect,
         choices=(
-            ('mark_refunded', _('Mark the complete order as refunded. The order will be canceled and all tickets will '
-                                'no longer work. This can not be reverted.')),
+            ('mark_refunded', _('Cancel the order. All tickets will no longer work. This can not be reverted.')),
             ('mark_pending', _('Mark the order as pending and allow the user to pay the open amount with another '
                                'payment method.')),
             ('do_nothing', _('Do nothing and keep the order as it is.')),
@@ -391,7 +390,7 @@ class OrderRefundForm(forms.Form):
         self.order = kwargs.pop('order')
         super().__init__(*args, **kwargs)
         change_decimal_field(self.fields['partial_amount'], self.order.event.currency)
-        if self.order.status in (Order.STATUS_REFUNDED, Order.STATUS_CANCELED):
+        if self.order.status == Order.STATUS_CANCELED:
             del self.fields['action']
 
     def clean_partial_amount(self):