This enhances the logic of the various get_*_url() functions to be more

strict when they're willing to accept either an ID or an object.
A new function, _normalize_id(), checks if the value is an ID or an

instance (or subclass) of the allowed class type. If it's not any of

these things, then a ValueError will be raised.
This reduces the complexity of the code in the functions, prevents

duplication, and adds enhanced error checking.

Unit tests pass.