Update health check to support new search endpoints

https://www.pivotaltracker.com/story/show/67321636

This changes the health check component so that the JSON client now supports the "unified" and "combined" search endpoints provided by Rummager. 

It drops support for the old-style Content API `/search.json` endpoint which was not used, and drops support for checking an individual index. This is because indexes are no longer queried on an individual basis - rather, weightings and filters are applied at the new endpoints.