Split schema config to support multiple doctypes

- Split the elasticsearch schema config file down into multiple smaller files and merge them together in Ruby.
- Load schemas for multiple doctypes and merge the doctype schema properties together with some core schema properties.
- Support `DELETE` requests for arbitrary doctypes.

I've got a [branch on specialist-publisher](https://github.com/alphagov/specialist-publisher/compare/rummager-integration) which integrates with these changes and successfully adds and removes documents from rummager when publishing and withdrawing.