circe-channel-user-nick-regain-p: New function.

This is used in two different places to make it clear that a heuristic
is used there. The new implementation does not really improve
the heuristic, just improves readability, but we might improve it later
on.
circe-channel-rename-user: Handle another case for activity.

When the recently-parted nick wasn't active, but the old nick
was, copy the last-active value, too.