Per r90849, factor out most of the code that's duplicated between Parser::getExternal...

Per r90849, factor out most of the code that's duplicated between Parser::getExternalLinkAttribs() and Skin::addToSidebarPlain() into wfMatchesDomainList(). Change a loose comparison to a strict one, and add a FIXME comment about how whitelisting nl.wikipedia.org also whitelists nds-nl.wikipedia.org due to the function's simplistic substring approach.