Fix IRC username case sensitivity

[cg505]

IRC usernames are not case sensitive, but in some cases, we assume they are. For instance, IRCBot.irc_users is a map from strings to IRCUsers,
where the keys are usernames. (example misuse) This cases an unhandled exception when you send the DM "Cooperc: test" to irc-bridge from slack.

We should look through the code and make sure we haven't made this mistake in other places as well.