Make customer identifier unique per organizer (#2647)

src/pretix/base/migrations/0215_customer_organizer_identifier_unique.py

@@ -0,0 +1,22 @@
+# Generated by Django 3.2.12 on 2022-05-12 15:34
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('pretixbase', '0214_customer_notes_ext_id'),
+    ]
+
+    operations = [
+        migrations.AlterField(
+            model_name='customer',
+            name='identifier',
+            field=models.CharField(db_index=True, max_length=190),
+        ),
+        migrations.AlterUniqueTogether(
+            name='customer',
+            unique_together={('organizer', 'email'), ('organizer', 'identifier')},
+        ),
+    ]