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.

Will this also change any values I've selected? eg when editing an unpublished draft, or if the form doesn't validate.