Rename "Data item" on client to "{{WBREPONAME}} item"

This changes the label of the link in the sidebar next to each
article that has a Wikidata item connected.

The German translation would change from "Datenobjekt" to
"Wikidata-Objekt". See https://de.wikipedia.org/wiki/Wikipedia:VV.

Change-Id: I6e1efcd3ca6b7bbf5c08096e553f8c06298dcc79