Site transition state (alternative)

To implement this cleanly, we've started recording when a site has a `special_redirect_strategy`. This lets us reflect sites where the organisation's supplier:
* redirects straight to www.gov.uk
* redirects to aka.organisation.gov.uk (which points at Bouncer)

The `special_redirect_strategy` will be populated by hand.

An alternative to https://github.com/alphagov/transition/pull/129