Fix login/logout redirects for sites with a URL prefix.

Out of the box, Mezzanine will redirect login/logouts to "/", which doesn't work great for sites with a prefix, such as www.foo.com/bar/. The rest of Mezzanine works fine in this setup, but upon login or logout, a redirect to www.foo.com occurs.

Change uses get_script_prefix() instead of "/" in order to play better with prefixes.