Combined search

Combine all the results needed for the GOV.UK site search into a single request.

This will let us pull out the hairy result-merging (and the multiple Rummager requests that go along with it) from [the controller in frontend](https://github.com/alphagov/frontend/blob/master/app/controllers/search_controller.rb#L11-105). It’ll also let us make the search work in a less hairy way without frontend needing to know about it.