Add support for using gone handler

www.agileplannerapp.com/boards/173808/cards/8116

This is required to return 410 response for routes setup using the [gone handler](https://github.com/alphagov/router#gone-handler). `content-store` sets up this route for [`gone items`](https://github.com/alphagov/content-store/compare/gone-route) to represent content that is no longer available.