Use IPageTemplate adapters for form templates

This change could be dangerous because i changed the Interfaces. Tries to allow 3rd party addons to customize templates without subclassing Form and FormWrapper of the ``plone.login`` and re-register in zcml on their own ThemeLayer.

This would be too complex, so the new approach is to just register a new form template. This was also mentioned here: https://github.com/zopefoundation/plone.z3cform/issues/4