High profile groups display on organisation index

Story: https://www.agileplannerapp.com/boards/105200/cards/4629

Many users are attempting to use the organisations index at www.gov.uk/government/organisations to find high profile groups which are not in their own right organisations. An example of this would be UKVI, where 8% of users landing on that page search for either 'visas' or 'immigration'. This adds high profile groups to the organisations index to satisfy this user need.

There are plans to re-think the design of that page / how users find organisations and this is only meant to be a quick-fix to satisfy current needs.

- [x] Passed product review
We should avoid mixing implicit parens here, eg `assert subject.high_profile_groups.is_a?(OrganisationsIndexPresenter)`