Introduce resource links in API responses

This change is for https://www.pivotaltracker.com/story/show/61656166

As seen in other API server apps (https://github.com/alphagov/gds-api-adapters/blob/062af656878e3aacd492b2ddaec77caed683f4aa/test/list_response_test.rb#L52-L55 https://github.com/alphagov/business-support-api/blob/master/app/views/business_support/search.json.erb) adds `links` Hash to tariff api responses. This enables automatic API traversal if needed.