allow BaseUser subclass to be used, which uses a UUID pk (#263)

piccolo_admin/endpoints.py

@@ -776,7 +776,7 @@ async def generate_file_url(
     def get_user(self, request: Request) -> UserResponseModel:
         return UserResponseModel(
             username=request.user.display_name,
-            user_id=request.user.user_id,
+            user_id=str(request.user.user_id),
         )
 
     ###########################################################################