Make vouchers reusable upon cancelation of an Order

Refs #165.

src/pretix/base/models/vouchers.py

@@ -162,10 +162,12 @@ class Voucher(LoggedModel):
 
     def is_ordered(self) -> int:
         """
-        Returns whether an order position exists that uses this voucher.
+        Returns whether a non-canceled order position exists that uses this voucher.
         """
         return OrderPosition.objects.filter(
             voucher=self
+        ).exclude(
+            order__status=Order.STATUS_CANCELLED
         ).exists()
 
     def is_in_cart(self) -> int:

src/pretix/base/services/orders.py

@@ -152,6 +152,11 @@ def cancel_order(order, user=None):
     if i:
         generate_cancellation(i)
 
+    for position in order.positions.all():
+        if position.voucher:
+            position.voucher.redeemed = False
+            position.voucher.save()
+
     return order