Fix regression from r26893: a few messages that formerly accepted character entities...

Fix regression from r26893: a few messages that formerly accepted character entities such as &nbsp; no longer did.  This broke existing behavior with no effective workaround, since a very common browser (Firefox < 3) cannot save forms containing literal nbsp (Mozilla bug 218277).  The messages (which are link text) now allow entities, but not wikitext or other HTML, via a new 'escapenoentities' option for wfMsgExt.