WebSocket: Add simple "refresh" event.

This is useful for when deploying updates to allow us to restart
everyone and get them on the new code.
Add "[live]" to page title if thread is live.
WebSocket: update title and sidebar when changed.
Add an unread update counter to the favicon.

This helps make it clear when new updates have come in while the
liveupdate stream was not focused.
Don't fetch activity pixels when page not visible.