Set $wgExportFromNamespaces to true on MediaWiki.org

This patch sets the $wgExportFromNamespaces setting
to true on MediaWiki.org while keeping it as false
for the rest of our wikis.

Please feel free to burninate this with fire if it
has potential for disaster, or even just for fun.

Bug: 59947
Change-Id: I684cab04df8d8fcd0d055bb2e63df64b5c91f0db