monitoring: Reorganize index structure

This patch improves the organization of the monitoring index page:

- Include the hostname (both OS and configured) for convenience.
- Round the uptime presentation for readability.
- Add a tiny CSS for consistency with the traces.
- Re-arrange the list of links for readability.

chasquid.go

@@ -91,7 +91,7 @@ func main() {
 	go signalHandler()
 
 	if conf.MonitoringAddress != "" {
-		launchMonitoringServer(conf.MonitoringAddress)
+		launchMonitoringServer(conf)
 	}
 
 	s := smtpsrv.NewServer()