Use 'http://' instead of protocol-relative '//' for stylesheets

Also, remove <!-- and --> comment delimiters.

Either change alone does not fix the issue, but the combination does.
Also, use stylesheet_link_tag instead of wicked_pdf_stylesheet_link_tag
since stylesheet_link_tag actually inlines the CSS in a <style> tag,
which appears to not work with the font-awesome library for some reason.