Represent content that's no longer available

**needs to be updated with the correct version of `gds_api_adapters` once https://github.com/alphagov/gds-api-adapters/pull/243 is merged**

www.agileplannerapp.com/boards/173808/cards/8116

Implementing [Gone items](https://github.com/alphagov/content-store/blob/master/doc/gone_item.md) in content-store to allow representing items that are no longer available. Also updates related documentation.
This is almost an exact replica of `RegisterableRoute`. How about subclassing it to reduce duplication:

```ruby
class RegisterableGoneRoute < RegisterableRoute
  def register!(rendering_app)
    Rails.application.router_api.add_gone_route(path, type)
  end
end
```

Also by making the signature the same, the additional `elsif` in `RegisterableRouteSet` [here](https://github.com/alphagov/content-store/blob/gone-route/app/models/registerable_route_set.rb#L53-L54) becomes redundant - the `RegisterableRouteSet` doesn't have to ask what type of routes they are, it just tells them to register.