redirect from /login should be an absolute redirect.

Following the docs here:

https://training.plone.org/plone-deployment/edit-project.html#activate-and-configure-the-add-on

visit the classic-ui and go to http://localhost:8080/plonesite/login/

This will give _relative_ redirect to authomatic-handler:

```
curl -i localhost:8080/RFAQSL/login/
HTTP/1.1 302 Found
Content-Length: 0
Date: Thu, 08 Dec 2022 13:36:04 GMT
Location: authomatic-handler
Server: waitress
Via: waitress
X-Frame-Options: SAMEORIGIN
X-Powered-By: Zope (www.zope.dev), Python (www.python.org)
```

And land you at http://localhost:8080/plonesite/login/authmatic-handler and give you a 404

However, note that the user experience when you click "login" in the classic ui you get a modal showing http://localhost:8080/plonesite/authmatic-handler

This is a bit strange, and I'm not sure what behavior is correct. But I think that the http://localhost:8080/plonesite/login url should do a redirect to an absolute path, and then make sure that the modal still works.