Automatically add `http://` to urls that lack a protocol in links.

We don't currently do this on v3, but the regularity with which I see
shows that have entered URLs that are interpreted as relative
(like http://prx.org/p/123945 – scroll to the bottom) is bothering me
right now, so I figure we might as well fix it for them rather than
lament their "doing it wrong".
for some reason this code kept being removed at commit time.
Merge pull request #97 from PRX/fix/related_site

Automatically add `http://` to urls that lack a protocol in links.