mediawiki.util: Optimise logic in addPortletLink

Optimised to no longer:
* boolean cast "nextnode" twice.
* assert nextnode.jquery when we know it is a jQuery object.
  > !!$( Node ).jquery
* assert nextnode.length when we know it has only 1 element.
  > $( Node ).length === 1
* execute $ul.find( selector ) twice for the CSS selector case.
* check for .length in some places and do an ignorant eq(0) in
  other cases (thus ignoring the case if there are multiple
  elements).

Tests still pass :-)

Change-Id: Ibc86c2dc33a8a3ba378138525c2523ba69bca8f1