Scope 'Curated Feature' links in exhibit nav to published pages only.

@cbeer I defer to you on this.  I noticed that #117 only showed published child pages in the sidebar of feature pages so I thought that we should mimic that behavior in the top level nav.  

This is a random PR that you very well may already be working on so feel free to close this w/o merging if you already have code to do this.