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/event.py

@@ -495,6 +495,22 @@ class Event(EventMixin, LoggedModel):
                 renderers[pp.identifier] = pp
         return renderers
 
+    def get_data_shredders(self) -> dict:
+        """
+        Returns a dictionary of initialized data shredders mapped by their identifiers.
+        """
+        from ..signals import register_data_shredders
+
+        responses = register_data_shredders.send(self)
+        renderers = {}
+        for receiver, response in responses:
+            if not isinstance(response, list):
+                response = [response]
+            for p in response:
+                pp = p(self)
+                renderers[pp.identifier] = pp
+        return renderers
+
     @property
     def invoice_renderer(self):
         """