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.
Ah, I wasn't sure whether the backend should be registered in all case or not. Why is it skipped for gone routes? I guess a guard on that line would suffice, e.g. `register_backend unless is_gone`. That might also make it more obvious that skipping it for gone routes is important.