Allow to set redirect URL after logout

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

It should be possible 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
• as the logout also logs the user out of all other Open-ID sessions, the Open-ID provider might provide a generic logout page, but still honoring the redirect_after_logout parameter. Configuring the parameter in XWiki would allow to redirect to the "common logout page".
• also users reported that their OpenID provider is rather restrictive what it allows as "redirect_after_logout" and limits it to certain fixed URLs; listing every page in XWiki for that provider is clearly impracticable.