Seat-specific vouchers (#1486)

* Basic functionality

* API

* Do not delete seats with vouchers

* Show seat in list of seats

* Validate availability of seats

* Fix invalid logic in Seat.is_available

* Show voucher name in edit form

src/pretix/base/migrations/0140_voucher_seat.py

@@ -0,0 +1,19 @@
+# Generated by Django 2.2.4 on 2019-11-14 11:49
+
+import django.db.models.deletion
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('pretixbase', '0139_auto_20191019_1317'),
+    ]
+
+    operations = [
+        migrations.AddField(
+            model_name='voucher',
+            name='seat',
+            field=models.ForeignKey(null=True, on_delete=django.db.models.deletion.PROTECT, related_name='vouchers', to='pretixbase.Seat'),
+        ),
+    ]