Update Interwiki::getURL's first argument so that it's properly urlencoded when repla...

Update Interwiki::getURL's first argument so that it's properly urlencoded when replacements are made.
Scanning all of core and all the extensions we have in svn, it doesn't look like anyone makes any calls to Interwiki::getURL using the argument so this should be safe enough to clean up the api.
Also update Title::getFullURL to make use of the first arg now.