Add gettingstarted-welcome-back-site-user and tweak anon msg.

* MediaWiki:gettingstarted-welcome-back-site-user is used for users who manually visit the GettingStarted page.
* The anon message is modified to simply be "Getting started" although the parameters remain available.

Change-Id: Ia60a5d1175cd7fa024b5388e7135d51c8ffb587c