Unpublish a website page

Takes a page back to a draft. Needs content: write, and is a write.

Its address stops answering at once and only the signed preview_url reaches it. published_at is left where it was, because it records when the page first went live rather than whether it is live now.

Unpublishing the newsletter page takes the whole website's article list away, and Commune goes back to linking the newsletter's name and avatar at its page on Commune. Nothing else on the site is affected.

Unpublishing a draft succeeds and changes nothing.

Publishes no event.