(bug 54492) Display a custom sitename for specific site groups

This will allow to specify a message for a sitename for
special sites as e.g. Wikimedia Commons which then gets
displayed instead of the language name in the first column
of the sitelinks table.

This requires to add the sitegroup to
$wgWBSettings['specialSiteLinkGroups'] and specify a message.

Needs https://gerrit.wikimedia.org/r/#/c/85986/ to be
merged into WikimediaMessages

Change-Id: Id6ba4e138794ae2aba8d2d96bc9a1154f2c38308
Merge "(bug 54492) Display a custom sitename for specific site groups"