Add "wgRelevantUserName" from skin to mw.config

A skin can have a relevant user, then some help links in the sidebar
are shown. When a user want extend this informationen with userjs, he
has to parse the existing items or the title param of the url to get the
name of the user for which this help links are shown. Having the name as
javascript variable makes it easier to add more links in the sidebar.

Change-Id: I17a75902b6e739d5149d332b6a94a6568b79501f
Merge "Add "wgRelevantUserName" from skin to mw.config"