Add a simple test mode (#1181)

- [x] Provide data model and configuration toggle
- [x] Allow to delete individual test orders
- [x] Add tests
- [x] Add a prominent warning message to the backend if test mode orders exist (even though test mode is off), as this leads to wrong statistics
- [x] Decide if and how to generate invoices for test orders as invoice numbers cannot be repeated or should not have gaps.
- [x] Decide if and how we expose test orders through the API, since our difference pull mechanism relies on the fact that orders cannot be deleted.
- [x] Decide if and how we want to couple test modes of payment providers?
- [ ] pretix.eu: Ignore test orders for billing
- [ ] Adjust payment providers: Mollie, bitpay, cash, fakepayment, sepadebit

![download](https://user-images.githubusercontent.com/64280/53009081-fe420d80-343a-11e9-8361-b8511c988598.png)

src/pretix/base/migrations/0111_auto_20190219_0949.py

@@ -0,0 +1,27 @@
+# Generated by Django 2.1.5 on 2019-02-19 09:49
+
+import django.db.models.deletion
+import jsonfallback.fields
+from django.db import migrations, models
+
+import pretix.base.models.fields
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('pretixbase', '0110_auto_20190219_1245'),
+    ]
+
+    operations = [
+        migrations.AddField(
+            model_name='order',
+            name='testmode',
+            field=models.BooleanField(default=False),
+        ),
+        migrations.AddField(
+            model_name='event',
+            name='testmode',
+            field=models.BooleanField(default=False),
+        ),
+    ]