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.

Using some `js-` prefixed classes should make this less fragile. If the markup changes the current JS could stop working.