make label-superevent pairs unique

gracedb/superevents/migrations/0002_superevent_label_unique_together.py

+# -*- coding: utf-8 -*-
+# Generated by Django 1.11.5 on 2018-04-11 18:30
+from __future__ import unicode_literals
+
+from django.db import migrations
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('events', '0018_event_superevent'),
+        ('superevents', '0001_initial'),
+    ]
+
+    operations = [
+        migrations.AlterUniqueTogether(
+            name='labelling',
+            unique_together=set([('superevent', 'label')]),
+        ),
+    ]