Added notifications for Comments.

Only goes to the Space Owner at this point, but it's progress.

Gave IdentityCache the ability to route messages to UserSessions.

Managed to screw up several times; added better error handling to
SpacePersistence to cope.
Tweak so that we don't wind up with a huge pile of messages marked unread.
Comment notifications now include a link to go to the comment.

This doesn't quite work yet, due to the timing of link resolution, but at
least the URL is hypothetically right.
Navigating to a specific comment now works.
Bugfix: UserValues that were set on the Space itself weren't showing up in the UI
You can now control whether you get Comment Notifications in this Space.

Via the Advanced page. It's primitive and hokey, and doesn't take effect
until the Space next reloads, but it's better than nothing.