Expand the protocol for proto-relative links when printing

Things like "User's Guide (//meta.wikimedia.org/wiki/Help:Contents)"
(on the default main page) don't look like the expected behavior to
me, and I think regular users expect links to start with either "www"
or "http".

Let's assume HTTPS for these and prepend the protocol to the URL.

Change-Id: If48f61de26a37eddbb134b824186706c08dabb72
Merge "Expand the protocol for proto-relative links when printing"