Google Code-in

[This is a slightly harder to task, you should have previously completed at least one MediaWiki programming task before attempting this one]

See https://phabricator.wikimedia.org/T183673 for the full details, but basically you need to

• Setup GlobalUsage locally with two wikis to test it.
• Copy includes/specials/Specialunusedimages.php into GlobalUsage extension
• Rename it to globallyUnusedFiles
• Change it to use the globalimagelinks table instead of imagelinks
• Make sure it only runs on the shared image repository.
• Change the various i18n messages
• Register the special page
• Test it

You are expected to provide a patch in Wikimedia Gerrit. See https://www.mediawiki.org/wiki/Gerrit/Tutorial for how to set up Git and Gerrit.