Add suggested queries to unified search

https://www.pivotaltracker.com/story/show/69085756

This adds a `suggested_queries` array to the unified search response. This contains spelling suggestions based on the existing `Suggester` class in Rummager.

We're calling this `suggesed_queries` rather than `spelling_suggestions` as we are considering providing suggestions based on other criteria in the future (such as common search queries for a particular facet/filter combination).