Search: Add Ranking Evaluation Integration

We've had a few requests about how to evaluate search rankings and improve the search algorithm for individual sites. Having a standard, defined way to help folks customize their search would help them get better relevancy.

This project looks like it could be good to integrate with but needs some evaluation: https://sease.io/2018/07/rated-ranking-evaluator.html

The tools there look a lot like what I used when [evaluating queries on the wp.org plugin directory](https://data.blog/2017/03/15/improving-relevance-and-elasticsearch-query-patterns/).

Some hooks we would need to figure out:
- how to get the base query from a site (I guess capture the search request)
- how to hook up to our query api for running the searches
- (eventually) how to pull in search analytics data to the tool

A very basic use case though is just testing and documentation of how to use the tools together with maybe some minor code changes to make it easier to get the search query info (maybe the existing integration with debug bar is sufficient though).

Start by evaluating the rated ranking evaluator tools and the existing Debug Bar integration mentioned in the issue. Investigate how to capture a site's base search query, connect it to the query API, and possibly incorporate search analytics. Done means establishing a basic testing and documentation path, with any minor query-information changes identified.

Written by the indexing model from the issue text.