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/models/orders.py

@@ -191,7 +191,10 @@ class Order(LoggedModel):
 
     @cached_property
     def meta_info_data(self):
-        return json.loads(self.meta_info)
+        try:
+            return json.loads(self.meta_info)
+        except TypeError:
+            return None
 
     @property
     def full_code(self):