privatemsg: Use thread id rather than message id

I was able to send a private message and receive it on my personal inbox (gmail). I can also reply to the message and it gets back to the private message inbox correctly to the right conversation and original sender. However, if a reply is sent from the private message inbox, a new conversation is created for some reason. At that point I cannot then reply from the external mailbox as the new conversation ID does not actually exist. This is the only issue I am having at the moment, but of course it is a big one as users right now can only send one round of messages.