Include organisations in the search index (and upgrade Rummageable) 

In order to supports future scoped search work, we're allowing mainstream content to be tagged with organisations in Panopticon.

This pull request includes a new field in `RegisterableArtefact` that is an array of organisation slugs, based on the tags associated with the artefact. 

To make this happen, Rummageable had to be updated from 0.3.0 to 1.0.0, which includes major changes to its public API. I've updated the RummageableArtefact class so that it continues to work, whilst abstracting some of the index configuration into a new class `SearchIndex`. I've also improved some of the unit test coverage for registering artefacts with Rummager.

This is dependent on the schema change made in alphagov/rummager#201.