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 think it would be helpful to extract this proc to a method with an appropriate name. It's a tricky one to name though... Maybe this line could become:

```ruby
  validates :title, :rendering_app, presence: true, if: :renderable_content?

  def renderable_content?
   !redirect? && !gone?
  end
```

Or maybe `live_content?`?