Allow additional interwiki prefixes on local interwiki links

This code was added by Brion back in 2004 as part of r6398.

According to Brion, links like [[en:wikt:foo]] on English Wikipedia are
"just crazy!", but I can't see any reason to disallow them.

Bug: 61357
Change-Id: If449e60a0ef95cfe3dd6f4b1809ac012be62a0ea
Merge "Allow additional interwiki prefixes on local interwiki links"