14. Enable and use publish workflow

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.