(bug 5149) recognize network-relative URIs from YT

Tweak the URI-matching regex to recognize either a scheme of http,
of https, or a schemeless network-relative URI (starts with //)
when it is coming from Youtube.
Merge pull request #471 from rickscott/bug5149-youtube-embed-rel-urls-a11y

(bug 5149) recognize network-relative URIs from YT