destroy user's registration requests on user delete

Author: mwa

qwc_config_db/config_models.py

@@ -184,3 +184,10 @@ def check_password(self, password):
         RegistrationRequest.registrable_group = relationship(
             RegistrableGroup
         )
+
+        # user registration requests
+        User.registration_requests = relationship(
+            RegistrationRequest,
+            # destroy registration requests on user delete
+            cascade="save-update, merge, delete"
+        )