SimpleSiteLink: make badges point to Items

Per decision made by the Wikidata team. Previously badges were
just strings.

PS 3: Update documentation for getBadges()
PS 7: Updated to use the new ItemId

Bug: 40810
Change-Id: I12339d01efe53e9226a7a3337d3691da4ee247e7
Merge "SimpleSiteLink: make badges point to Items"