Refactor and lazy load jquery.wikibase.linkitem

While I was on it I changed wbclient.LinkItem into a jQuery
widget (that's why I had to rename it). This makes the whole
thing much more flexible.

Change-Id: I9a03c3d974f181e8eadaa30b673e7c662033cb1c
Merge "Refactor and lazy load jquery.wikibase.linkitem"