Add typeahead support for selecting the featured image of a browse category

Fixes #546 

Change from storing the URL of a featured image to storing the solr document ID of a featured item.

`featured_image` still exists and returns the configured thumbnail url of the `featured_item`.

![browse-typeahead](https://cloud.githubusercontent.com/assets/96776/2597309/4d77c2e4-bab2-11e3-85a2-97b652c40312.png)