Add PerPageLicense extension.

See https://www.mediawiki.org/wiki/Extension:PerPageLicense
The PerPageLicense extension enables license footer variables
(viz. $wgRightsIcon, $wgRightsUrl, and $wgRightsText) to be
set on a per-namespace or per-page basis.

Change-Id: I9a211eea021b61c7768e2fcd0bbd6bc0111c9af8