Allow to set an explicit redirect URL after logout

Similar to OIDC-205, but independent of the current authenticator:

Currently after logout the browser is redirected to the page from which the user clicked the "Logout" link.

It would be nice to optionally allow to redirect to another URL instead (via configuration, not via request parameters). Use Cases:

• the current page might be view protected and the user should be redirected to a public wiki page (instead of the login form, where it might be tricky to navigate back to the wiki)
• some users prefer if they get a feedback like "you have been logged out" after logging out
• also some authenticators might require an explicit "logout page"

This can be obtained by setting the "xredirect" request parameter of the logout link (currently in drawer.vm) to that configured value instead of the URL of the current page.