Add Facia signposting override for editorally curated index pages

This will allow the editors to curate a list of our most popular contributors and subjects through Facia tool, which will have the full a-z index pages in the nav.

![screen shot 2014-09-04 at 11 50 51](https://cloud.githubusercontent.com/assets/1001642/4148146/72f9346a-3421-11e4-9120-62e1d7bf8d41.png)