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.
I like this approach, however `RegisterableRouteSet` would still need to query the item type to determine whether or not to register the backend.