findUrl: don't match non-:// URLs

Although "x:5" could theoretically be a URL with scheme "x" and path
"5", it probably isn't. Only URLify strings that use the "authority"
syntax ("foo://"). (No one ever types out "mailto:" URLs in ordinary
text, so we don't want to match those either.)

https://bugzilla.gnome.org/show_bug.cgi?id=636252