No unpublish side effects

[cavis]

For PRX/publish.prx.org#698.

This prevents 2 bad cases, when updating story.published_at to match the story.released_at.

1. If published (published_at <= now) ... don't allow nulling out published_at.
2. If published (published_at <= now) ... don't allow setting published_at > now.

Some fallout on the frontend (PRX/publish.prx.org#702). But I think we always want to prevent these 2 cases on the backend in any case. The only API path to unpublishing something is POST /api/v1/stories/1234/unpublish.

[svevang]

Looks good, just found one side effect:

Given a scheduled story,
Select "publish" from the status dropdown
Click the "Publish Now" button
See the released_at date bounded to todays date
Click 'edit' next Dropdate and set a date far in the future
Click the "Publish Now" button
See a 409 error message
Try to make an unrelated change, e.g. to the title
Click the "Publish Now" button
See a 409 error message
Reload the page and see the released_at date is still far in the future

At this point, it seems I'm caught in a loop. Digging into the localstorage, it seems that the publish/unpublish box is holding onto the released_at attribute of the Story. If I clear out the stories from the localstorage, then the publish released at box shows the released_at date bounded with the published date.

Furthermore, checking out the console, the released_at attribute is nil throughout this process.

[cavis]

You're onboard with the CMS logic in this PR then?

Yeah, that's not great. Let's not merge this yet then, and I'll try to clean that up in PRX/publish.prx.org#702, and we can do a concurrent deploy. (I do like CMS doing the "right thing" to prevent this).