admindb should have 'logout' function

[Tokio Kikuchi]

Current 2.1 admindb interface have no link (or button) to logout the administrator/moderator. An administrator can logout from mailman from admin interface but a moderator cannot logout without zapping the moderator cookie by browser's function (if it is provided) or terminating the browser. The admindb web page should have a convenient 'logout' link.
Another inconvenience in admin logout funciton is that if the site-wide admin is allowed by mm_cfg.ALLOW_SITE_ADMIN_COOKIES then the administrator cannot logout with visiting the 'Logout' link in the admin page.
These bugs are fixed by lp:~tkikuchi/mailman/logout-enforcement and the branch was requested to merge into 2.1 series.