Lurker predicate

[defanor]

Detecting whether a user is active (and not a lurker) by the time
passed after their last message doesn't work well for low-traffic
channels, where a conversation participant should still be considered
active if their last messages are not far in the log (and possibly
still on the screen, even). So a lurker predicate should be a
disjunction of those two: one for time, and one for the amount of
messages.

[jorgenschaefer]

Sounds good.