Use "protocol-relative URLs" for internal links

A protocol-relative URL is one that omits the http: or https: part of
the URL (see http://stackoverflow.com/q/4978235/ for details), and thus
will use the same protocol as the one used to load the current page.
This should solve the problem of, say, embedded Youtube videos not
showing up when the page is loaded by https (because the browser sees an
http embed on an https page and disables it for security reasons).