Allow selecting the same add-on multiple times (#1717)

src/pretix/base/migrations/0157_auto_20200712_0932.py

@@ -0,0 +1,20 @@
+# Generated by Django 3.0.6 on 2020-07-12 09:32
+
+from django.db import migrations, models
+
+import pretix.helpers.countries
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('pretixbase', '0156_cartposition_override_tax_rate'),
+    ]
+
+    operations = [
+        migrations.AddField(
+            model_name='itemaddon',
+            name='multi_allowed',
+            field=models.BooleanField(default=False),
+        ),
+    ]