Make Login form show both, Authomatic and Local logins form

At the moment the classic login form is not shown by default. This is because of a Plone "feature: if the registry setting `plone.external_login_url` is set to something the default form is replaced by what the given url delivers.

We need some way to support both types of forms. But imo this is a Plone core problem, not a problem of this plugin. So we can workaround here or tackle the problem upstream.

At the moment the `failsafe_login_form` works as fallback.