Fix bug where login page redirects to wrong place

When logging in to the Jamaican Psalms site on the dev server, the
redirect was going to dev.scriptureforge.org instead of to the correct
jamaicanpsalms.dev.scriptureforge.org URL. This is because the
redirect() function, if given a relative URL, looks up HTTP_HOST to
flesh out the URL -- and on the dev server, HTTP_HOST is always going to
be dev.scriptureforge.org because of the way our reverse proxy is set up.