Added migration

src/core/migrations/0057_UniqueTogether2EventParticipant.py

+# Generated by Django 3.1.4 on 2020-12-29 12:05
+
+from django.db import migrations
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('core', '0056_backoffice_users_perms'),
+    ]
+
+    operations = [
+        migrations.AlterUniqueTogether(
+            name='eventparticipant',
+            unique_together={('participant', 'event', 'role')},
+        ),
+    ]