Added optional kwargs to get_expiry_age/date.

This change allows for cleaner tests: we can test the exact output.

Refs #18194: this change makes it possible to compute session expiry
dates at times other than when the session is saved.

Fixed #18458: the existence of the `modification` kwarg implies that you
must pass it to get_expiry_age/date if you call these functions outside
of a short request - response cycle (the intended use case).