Warn in the Extension Manager & Distribution Wizard UIs when *.xwiki.org is down for maintenance

From time to time we need to do maintenance operations on the xwiki.org wiki instance (affecting extensions.xwiki.org), or nexus.xwiki.org (maven repo).

It would be nice to be able to set a maintenance message on e.x.o somewhere and have the XWiki instances in the world, display this message in the DW UI and in the EM UI, and prevent upgrading or installing extensions during this time.