Use "article" instead of "page" only on Wikipedia

* Revert to using "page" by default
* Use the MessageCache::get hook to rewrite the keys for a subset of
  GettingStarted messages so that the resulting message uses "article"
  instead of "page"
* Only register the MessageCache::get hook on Wikipedia

Change-Id: I3ea11c0b584039bed051fe9bdd2a18e3d1ef6210