Dynamic quit messages, take 2

Let's see if I can do this without messing up my local repo and accidentally introducing unrelated commits.

This was suggested by Iciloo. It simply converts the !quit message from the static "died of an unknown disease" to a random choice from a list, similar to the lynch messages. Pretty minor, I know, but it spices things up a bit.

Has been tested on a current codebase with no evident problems.

I thought of changing the disconnected messages as well, but those happen infrequently enough that I don't see any particular improvement to having them be random.
variable names are inconsistent with what is already in settings, e.g. LYNCH_MESSAGES / LYNCH_MESSAGES_NO_REVEAL. I recommend following the same convention instead of adopting different conventions for everything, because that's just confusing.