Publishing a Thing with embedded comments, where the comments aren't public, hangs

Summary: Yes, the recipe is that complex. Took me a week to track down...

The problem is that, if the Thing has embedded comments, it will try to fetch those comments when rendering the page for the change log for publication. But the render is currently done with the public permissions. (Because otherwise, trying to manage the permissions of Recent Changes becomes nightmarish.) So if the Who Can Read Comments doesn't cover the public, that will fail.

Compounding this, ThingConversationsCore returns the ThingError when the comments aren't readable. But the handlers for that message don't cope with ThingError. (Because, say it in unison, Akka is weakly typed.) So the ThingError crashes the receiver, and everything fails when it times out.

The fix is probably to return an empty ThingConversations instead of the error.