Have a way to show a service status message on the homepage

[edg2s]

Quite a few times we've had issues where a bug in core or an extension has prevented wikis from being created, most recently #471. As many of our users are non-technical it may be helpful to show a temporary banner on the homepage while these issues are ongoing so users are aware there may be a problem in creating a wiki, and notify them of any workarounds.