Don't show discussion links for links in private subreddits.

Fixes #19.
Infinite scroll: don't crash if no listing table present.
WebSocket: ensure connecting throbber isn't cut off.
WebSocket: don't use sockets for closed events.
Update for reworked core websocket support.
WebSocket: broadcast delete and strike events.
WebSocket: factor out websocket namespace in broadcasts.
WebSocket: send out periodic updates of active visitor counts.