Add FeinCMS navigation

Was initially a template tags https://github.com/incuna/incuna-feincms/blob/775ca31027807a17db10fbdc3f3ab8fde95ac4ef/incunafein/templatetags/incunafein_tags.py#L38-L264

It should be a template.
Is this a requirement? If so, please add it to the README.