Add $wgServerName

Add $wgServerName

This partially reverts r73950 which removed $wgServerName on the ground that it
was only used for {{SERVERNAME}}. When it was pointed out that $wgServerName was
also used by several extensions, the response was not to restore the variable, but
to proceed to remove it from extensions as well.

It is a useful variable to have, as the discussion on Id819246a9 makes clear
(see Tim's comment on PS12 and Timo's reply). So let's reintroduce it, and expose
it in mw.config and ApiQuerySiteInfo as well.

Change-Id: I40a6fd427d38c64c628f70a2f407b145443ea204