Add database-level uniqueness constraint for check-ins

We measured that this creates a ~10% performance loss on MySQL, but
believe that correctness is more important. Also, in case on concurrent
check-ins on MySQL with default transaction isolation level, this might
lead to Internal Server Errors on all but one check-ins, which is still
better than to show green.

src/pretix/base/migrations/0112_auto_20190304_1726.py

@@ -0,0 +1,27 @@
+# Generated by Django 2.1.5 on 2019-03-04 17:26
+
+from django.db import migrations
+from django.db.models import Count
+
+
+def make_checkins_unique(apps, se):
+    Checkin = apps.get_model('pretixbase', 'Checkin')
+    for d in Checkin.objects.order_by().values('list_id', 'position_id').annotate(c=Count('id')).filter(c__gt=1):
+        for c in Checkin.objects.filter(list_id=d['list_id'], position_id=d['position_id'])[:d['c'] - 1]:
+            c.delete()
+
+
+class Migration(migrations.Migration):
+    dependencies = [
+        ('pretixbase', '0111_auto_20190219_0949'),
+    ]
+
+    operations = [
+        migrations.RunPython(
+            make_checkins_unique, migrations.RunPython.noop,
+        ),
+        migrations.AlterUniqueTogether(
+            name='checkin',
+            unique_together={('list', 'position')},
+        ),
+    ]

src/pretix/base/models/checkin.py

@@ -167,6 +167,9 @@ class Checkin(models.Model):
         'pretixbase.CheckinList', related_name='checkins', on_delete=models.PROTECT,
     )
 
+    class Meta:
+        unique_together = (('list', 'position'),)
+
     def __repr__(self):
         return "<Checkin: pos {} on list '{}' at {}>".format(
             self.position, self.list, self.datetime