Added Person#without_a_current_ministerial_role

This replaces and clarifies the intention of Person#non_current_ministerial_roles, and provides a test.
Change Searchable to respect :only param in update_in_search_index

Previously searchable_options[:only] was only used to scope the objects retrieved when using Class.search_index to index all the instances.  This meant if we only wanted to index some instances we had to deal with it in our own callbacks for normal lifecycle events.  Now the only param is used as a guard (by adding .find_by_id(self.id).present?) inside update_in_search_index so that if the instance is not present in the scope it won't be added to the search index.
Corporate info pages belonging to non-live orgs are not search indexed
Data migration to remove corp info pages and people wrongly indexed
Merge pull request #337 from alphagov/only-index-corp-info-pages-belonging-to-live-orgs

Only index corp info pages belonging to live orgs

https://www.pivotaltracker.com/story/show/45957935