Extend API to support editing badges

Adds ability to edit badges through wbeditentity
and wbsetsitelinks API modules.

This change makes link title optional provided that
badges are set. Otherwise even if one only wanted to
modify badges (s)he'd have to know the current page
title.

The link is deleted either if page name is an empty
string or both page name and badges are not set.

Bug: 40810

Change-Id: I27e9c5e3e86ed04813e60787abe0e0856f2893b3