Comments from Guest Users cause havoc

Summary: Eric reported that both Notifications and some comment-displaying pages in the Spirit Island Playtest Space were failing. On looking at the logs, this appears to be the problem.

The issue is pretty straightforward: the Conversation code assumes that, when rendering a Comment, we fetch the Identity of its author as part of that. But Guest Users don't have an Identity, intentionally -- they exist only as a pseudo-Identity with rights only inside one Space. So we need to render them as "Guest User", consistent with how we handle it otherwise.