use https:// rather than // for link urls

use https:// rather than // for link urls

this allows us to view more of the assets when opening up each page in a
browser, without using a web server. previously, with the //... urls and
pages open as file://..., the links would break. we have no need for //
because we have standardized on https for most of our sites, including
static.fsf.org and weblabels.fsf.org. // is only useful if we want to
use http rather than https some of the time.