Allow per-user logoutURL

[ffdixon]

Is your feature request related to a problem? Please describe.
Front-end applications may have users enter through different paths in the application, and when the user log's out want to return them to their previous login screen.

Currently, the logoutURL is shared by all users and doesn't provide any way to redirect different users to different areas.

Describe the solution you'd like
In the join API parameters, add support for a logoutURL to enable the front-end to send each user their own logout URL.

Describe alternatives you've considered
We could add parameters to the logoutURL on create, such as %%USERID%% or %%ROLE%%, but giving each user their own logoutURL is simpler and gives all the control to the front-end.

[ffdixon]

This will also cover the case where the front-end wants the users to logout to completely different URLs as well.