Define MediaWiki:GlobalUserPage.css in i18n file

This way it is shown on Special:AllMessages
Follow up to Ib31523bd973277ff3f8a86dbe96108a88cdc9975

Since that page only exists on the central wiki,
only load the messages there, like we do in GlobalCssJs.

Change-Id: I8f3ecdd7bb49df9f0098826b55371c9a692f9716
Merge "Define MediaWiki:GlobalUserPage.css in i18n file"