Primary site logo has leading slash inserted breaking remote logo image links

Now have:

    {{ h.url_for_static(g.site_logo) }}

url_for_static inserts leading slash which then breaks logos which are remote e.g. http://assets.okfn.org/p/ckan/img/ckan_logo_box.png

which becomes

http://localhost:5000/http://assets.okfn.org/p/ckan/img/ckan_logo_box.png

Suggest either do not use `url_for_static` or change that method to check whether url starts with http:// or https:// (or // even ...)