Fix #678 -- Data shredders for personally identifiable information (#817)

* Add data shredders for PII

* First working shredder

* Add more shredders

* Add new shredders and download confirmation

* tmp

* PayPal, Stripe, banktransfer

* Add icon to logs

* Untested payment log shredders

* Add waiting list shredder

* First tests

* Add tests for shredders

* Improve templats, link to shredder

* Test payment info shredders

* More tests

* Documentation

* Fix enabled flag in payment provider overview

* Fix minor issues

src/pretix/base/migrations/0089_auto_20180315_1322.py

@@ -0,0 +1,25 @@
+# -*- coding: utf-8 -*-
+# Generated by Django 1.11.10 on 2018-03-15 13:22
+from __future__ import unicode_literals
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('pretixbase', '0088_auto_20180328_1217'),
+    ]
+
+    operations = [
+        migrations.AddField(
+            model_name='logentry',
+            name='shredded',
+            field=models.BooleanField(default=False),
+        ),
+        migrations.AddField(
+            model_name='invoice',
+            name='shredded',
+            field=models.BooleanField(default=False),
+        ),
+    ]