Add a configuration option to redirect to after logout

[RKrahl]

As far as I understand the procedure (correct me if I'm wrong), TopCAT does the following on logout:

1. Get the ICAT sessionId from browser session storage
2. Logout from ICAT
3. Delete the session from browser session storage
4. Redirect to the TopCAT login page

If the login was done using an external identity provider such as Keycloak, we need to terminate the session there as well. This could easily be done if we could configure the URL that TopCAT redirects to in the last step.

E.g. the default is the TopCAT login page that is something like https://topcat.example.org/. In my use case, I want to manage the OpenID Connect session in an Apache reverse proxy in front of TopCAT using mod_auth_openidc, I would need a redirect to something like https://topcat.example.org/auth/redirect_uri?logout=https%3A%2F%2Ftopcat.example.org%2F [edit: the parameter needs to URL encoded of course]. That would, after terminating the session in mod_auth_openidc, eventually redirect once again to the TopCAT login page.

Therefore, I'd like to ask to add a configuration option for the URL to redirect to on logout. It would need to be a static URL that may include a parameter.