First pass on search for service manual

The service manual team want to add search for the manual. We're experimenting with a few approaches to that, one of which is to use a new index in rummager. This branch:

* Adds ```rake rummager:index``` that takes any JSON files in the ```db/index``` folder and pushes their contents to a rummager index called 'specialist'
* Provides a route of ```/service-manual/testsearch``` for reviewing results. This is deliberately not /search so that other search approaches can be tested simultaneously and/or a "sorry we don't have search" page can exist as at present
* Adds a feature flag that can be used to hide search until we're happy launching it

**This code is not tested**. It began as a proof of concept but now needs testing in a preview environment. I don't have time to retrospectively add the tests at the moment, so if anyone else would like to that would be great. At the very least it needs to be clear that this code doesn't deal with a few error conditions that will need to be considered before this goes to production. Maintaining the "testsearch" URL is part of highlighting that.

To work well this relies on https://github.com/alphagov/government-service-design-manual/pull/43

To work in deployment it will also need the corresponding change in alphagov-deployment: https://github.com/alphagov/alphagov-deployment/pull/120

In order to deploy this it's also necessary to create the new search index **before** deploying. Instructions for that are in: https://github.com/alphagov/rummager/blob/master/README.md - there's a corresponding pull request in rummager at https://github.com/alphagov/rummager/pull/76