Add last_modified property to orders (#907)

src/pretix/base/migrations/0091_auto_20180513_1641.py

@@ -0,0 +1,20 @@
+# -*- coding: utf-8 -*-
+# Generated by Django 1.11.11 on 2018-05-13 16:41
+from __future__ import unicode_literals
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('pretixbase', '0090_auto_20180509_0917'),
+    ]
+
+    operations = [
+        migrations.AddField(
+            model_name='order',
+            name='last_modified',
+            field=models.DateTimeField(auto_now=True, db_index=True),
+        ),
+    ]