Use feature to properly control Worldly editions in searchable_classes

E.g. WorldLocationNewsArticle and WorldwidePriority are always in edition_classes and we remove them from searchable_classes only if the world_feature? flag is false.
WorldLocations are now Searchable

Index title and mission statement, only for active locations and only if the world_feature? flag is true.
WorldwideOrganisations are now searchable

Indexes name and summary for all worldwide orgs, but only if the world_feature? flag is true.
Add world-location-news to list of edition_route_path_segments

This so that govspeak will properly rewrite admin links to their public counterpart.
Add more tests around Whitehall.government_search_index contents

Particularly the stuff that is only present depending on the world_feature? flag.
Merge pull request #373 from alphagov/search-more-world-things

Search more world things

https://www.pivotaltracker.com/projects/367813#!/stories/39934581