Factor out skins from templates

[rob-metalinkage]

The templates duplicate boilerplate skins making customisations rely on full copies - which means improvements wont flow to customisations.
Templates should have a fragments inclusion model so headers, footers and styles get included - or templates themselves are fragments included into a master template (like django templates extend a base) so custom skins only need to be defined once.

[nicholascar]

I think this is addressed now: the OGC theming repo only have a template for index.html - font page content - and page.html site-wide styling.