Rename mainstream_content_api to content_api

It better reflects what it is and what it's for.
Use Whitehall.content_api from SpecialistSector

Also stub out the tags method it uses.
Use real content_api for specialist sector tests

Whitehall.content_api is a faked double in development and test
environments. For the specialist sector tests we explicitly want to test
against a real content_api adapter with stubbed web requests.