Support 410 responses

At the moment, if an app using slimmer returns a 410 response, the status code
is preserved, but the body is rewritten to the generic 500 error page.

Whilst I believe pages which have been removed from GOV.UK should be registered as such
with the Router, if an app returns a 410, it should be honoured. Additionally, if Slimmer is reused without the Router by third parties, then responses should not be rewritten to look like server errors.

Static already has a 410 template, so this should "just work". I tested it successfully in development.