Cancel a scheduled article

Takes a queued article back off the schedule. Needs sending: write.

The article returns to draft and its send time is cleared, so nothing will dispatch it until it is scheduled or sent again. It was never visible to readers while it was queued, so nothing a reader sees changes.

An article that is not scheduled answers 422 rather than succeeding quietly. draft and sent are both "not scheduled", and a caller cancelling a send needs to know which one it is looking at: an article that has already gone out cannot be recalled.

Publishes no event. A consumer told the article was scheduled gets no second event saying it no longer is, so read status on the article to know where it stands.