Handle 'health_status' container event

This avoids an obsolete UI state when a new health check result is
available, and silences an "Unknown event type" console warning. Before
this was covered up by listening to lots of other (unrelated) events,
which mostly (but not always!) update the UI anyway to show the new
health event.

src/app.jsx

@@ -352,6 +352,7 @@ class Application extends React.Component {
         case 'create':
         case 'died':
         case 'exec_died':
+        case 'health_status':
         case 'kill':
         case 'cleanup':
         case 'mount':