Allow quota-level vouchers for hidden products (#1123)

* Changes in checks

* Backwards-compatible implementation

* Add test

* Fix voucher bulk form

src/pretix/base/migrations/0125_voucher_show_hidden_items.py

@@ -0,0 +1,26 @@
+# Generated by Django 2.2.1 on 2019-07-07 10:10
+
+from django.db import migrations, models
+
+
+def set_show_hidden_items(apps, schema_editor):
+    Voucher = apps.get_model('pretixbase', 'Voucher')
+    Voucher.objects.filter(quota__isnull=False).update(show_hidden_items=False)
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('pretixbase', '0124_seat_seat_guid'),
+    ]
+
+    operations = [
+        migrations.AddField(
+            model_name='voucher',
+            name='show_hidden_items',
+            field=models.BooleanField(default=True),
+        ),
+        migrations.RunPython(
+            set_show_hidden_items,
+            migrations.RunPython.noop,
+        )
+    ]