Fix #293 -- Add ticket downloading reminder (#567)

Closes https://github.com/pretix/pretix/issues/293

src/pretix/base/migrations/0072_order_download_reminder_sent.py

@@ -0,0 +1,20 @@
+# -*- coding: utf-8 -*-
+# Generated by Django 1.11.4 on 2017-08-04 13:42
+from __future__ import unicode_literals
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('pretixbase', '0071_auto_20170729_1616'),
+    ]
+
+    operations = [
+        migrations.AddField(
+            model_name='order',
+            name='download_reminder_sent',
+            field=models.BooleanField(default=False),
+        ),
+    ]