14. Enable and use publish workflow

[dbu]

The publish workflow is already there, but we need to enable it for the admin and respect it when looping over content.

As our content extends the simple cms Page document, it already implements PublishableInterface and PublishTimePeriodInterface. We chose which things we want to edit on each content.

Direct URLs are checked automatically (open a presentation, unpublish it, hit reload and you get a 404). But when we loop over lists of content, we need to check cmf_is_published() before displaying content.