This allows to redirect the user to the procedure they signed up for
even when the browser session is not available (like if they changed
of browser).
Fix#4738
Refactored from 6a69d958da
The `after_confirmation_path_for` isn't really made to be a callbback.
For instance, it is not executed during tests.
Moving the invitations linking to a proper documented callback allows
the linking to work in a testing environment, when invoking `user.confirm`.
