Ask about a document's origin

https://www.agileplannerapp.com/boards/173808/cards/5431

Implement prototype from [prototype-first-published](prototype-first-published): add compulsory radio choice with "this document is new" vs "previously published on another website", and if the latter then show a compulsory first_published date selector.

I think this would be more helpful for future-developer as several tests describing the specific cases. e.g. `#previously_published returns true for an imported edition, regardless of the first_published_at timestamp`, `#previously_published returns nil for a new edition`, `#previously_published returns true for an edition with a first_published_at timestamp`.