Monolithic commit re-factors OrganisationType and adds a Devolved Administration type.

• OrganisationType replaced with a static (non-database) model
• OrganisationType related stuff extracted from Organisation into a separate concern.
• OrganisationsController#index cleaned up - all organisations are retrieved with a single query and logic for grouping moved into a new OrganisationsIndexPresenter.
• Lots of updated tests.# This is a combination of 3 commits.
Migration to add new devolved administration organisations.
OrganisationType agency_or_public_body and non_departmental_public_body are now model attributes.
Bring #some# sanity to organisations/index view.
Added devolved administrations to organisations index view.
Validating that devolved administrations are govuk_status exempt.