(bug 29497) To support protocol-relative URLs in external links, add '//' to $wgUrlPr...

(bug 29497) To support protocol-relative URLs in external links, add '//' to $wgUrlProtocols and remove a useless \b in the bracketed external link regex in the parser that prevented the former from working outright. With this change, things like [//example.com/wiki/Foo Bar] are now supported, and [[{fullurl:Foo|action=edit}} Bar] no longer breaks if $wgServer is set to '//example.com' but produces a protocol-relative link instead. Protocol-relative URLs in free text are not picked up and turned into [1] links like URLs with protcols are; this is probably due to another \b somewhere, but I think this is a good thing.