Fix #1521 -- External authenticated users cannot delete events (#1523)

* Remove check password for event deletion, instead require recent login.

* Reauthenticate for backends using authentication_url.

* Require recent login for data shredder and prompt slug instead of password.

* Fix tests for recent login required on event delete and data shred.

* Pull request remarks for recent login required for event delete and data shred.

* Remove unused imported check_password.

src/pretix/base/forms/auth.py

@@ -198,6 +198,7 @@ class ReauthForm(forms.Form):
         self.request = request
         self.user = user
         self.backend = backend
+        self.backend.url = backend.authentication_url(self.request)
         super().__init__(*args, **kwargs)
         for k, f in backend.login_form_fields.items():
             self.fields[k] = f