Add idempotenty nonces to pretixdroid API

src/pretix/base/migrations/0059_checkin_nonce.py

@@ -0,0 +1,20 @@
+# -*- coding: utf-8 -*-
+# Generated by Django 1.10.7 on 2017-05-04 07:06
+from __future__ import unicode_literals
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('pretixbase', '0058_auto_20170429_1020'),
+    ]
+
+    operations = [
+        migrations.AddField(
+            model_name='checkin',
+            name='nonce',
+            field=models.CharField(blank=True, max_length=190, null=True),
+        ),
+    ]

src/pretix/base/models/checkin.py

@@ -8,3 +8,4 @@ class Checkin(models.Model):
     """
     position = models.ForeignKey('pretixbase.OrderPosition', related_name='checkins')
     datetime = models.DateTimeField(default=now)
+    nonce = models.CharField(max_length=190, null=True, blank=True)