Handle self messages

23:54:36 shivaram> a "self message" is a message that was sent by a different client connected to your nickname; typically this is in a bouncer context (like ZNC) but with oragono the server will send them natively
23:55:18 shivaram> a self message has source/NUH corresponding to your own nickname, and target (the first parameter of PRIVMSG) equal to someone else's nickname
23:56:38 shivaram> it's essentially a problem of recognizing that the source of the message has nickname equal to one's own nickname, then redirecting it to the buffer/window corresponding to the nickname of the target, instead of the source

Reproduce that in oragono by “connecting” two IRC clients behind the same nick (we probably disabled this feature in the conf file, since we want to test the nick conflict thing. Re-enable this just for this scenario).