Add a maximum budget to vouchers (#1526)

* Data model changes

* Fix test failures

* Adjustments

* Some tests and API support

* Check when extending orders

* Make things more deterministic, fix style

* Do not apply negative discounts

* Update price_before_voucher on item/subevent changes

* Add tests for price_before_voucher in combination with free price

* Fix InvoiceAddress.DoesNotExist

src/pretix/base/migrations/0142_auto_20191215_1522.py

@@ -0,0 +1,30 @@
+# Generated by Django 2.2.7 on 2019-12-15 15:22
+
+from decimal import Decimal
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('pretixbase', '0141_seat_sorting_rank'),
+    ]
+
+    operations = [
+        migrations.AddField(
+            model_name='cartposition',
+            name='price_before_voucher',
+            field=models.DecimalField(decimal_places=2, max_digits=10, null=True),
+        ),
+        migrations.AddField(
+            model_name='orderposition',
+            name='price_before_voucher',
+            field=models.DecimalField(decimal_places=2, max_digits=10, null=True),
+        ),
+        migrations.AddField(
+            model_name='voucher',
+            name='budget',
+            field=models.DecimalField(decimal_places=2, max_digits=10, null=True),
+        ),
+    ]