SNN: display news item categories in the properties screen

If I recall, news items do not do the "new silva thing" where the document is
rendered on the edit tab of published items. If this was the case, the
subjects/ta's could be included in the metadata column of the news item. You
know, the column that lists the publication/display date/time and location.

That's a good point, the editor screen doesn't show the preview. But
the preview uses the public templates, and we can only show the
categories if they're included in the public template too.

Aha, what we can do is include an info box, like we have for images,
that lists the categories. Layout's going to be a bit tricky, but it's
possible.

Although I still wonder if we shouldn't make them appear/editable in
the properties screen. That's probably quite some work.

An info box was added on the edit screen of published news items. This works around the original issue (what the categories of published items are). Repurposing this as SilvaNews metadata (to go on the properties screen) may be a bit of work.