Allow conversations with peers that are not in the users list, and don't try to display presence for them

This has come out of some of the SPA work I've been doing, but also I think we're going to want it for instant share.

The idea here is that if a contact isn't in the users list (e.g. instant share), we should still start a conversation to them.

Additionally, I've also stopped the display of the presence information in the conversation window as it doesn't apply in this case.