Changed external http:// links to // protocol-independent paths

When serving CKAN on https://, browsers will complain when parts of a rendered page attempt to get data over http://. 

This actually prevents reCAPTCHA from rendering at all on a new user form when CKAN is on https://.  Luckily, most of the external services referenced in CKAN are available over http:// and https://.  This fix allows for the right protocol to be chosen automatically.