Fix #91 -- Make products/orders that are not cancelable (#204)

* Make products/orders that are not cancellable
Update Unit Tests

* Make use of select_related instead of prefetch_related
Simplify cancellable logic
Migrations

* Simplify cancellable logic
Maske use of select_related instead of prefetch_related

* Fix cancelable condition bug

src/pretix/base/migrations/0035_merge.py

@@ -0,0 +1,16 @@
+# -*- coding: utf-8 -*-
+# Generated by Django 1.9.9 on 2016-08-31 21:04
+from __future__ import unicode_literals
+
+from django.db import migrations
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('pretixbase', '0033_auto_20160822_1044'),
+        ('pretixbase', '0034_auto_20160830_1952'),
+    ]
+
+    operations = [
+    ]