Allow conversion of "guest" sessions to "logged in" sessions

[audiodude]

After a user has create a session as a "guest" (#32), we should also allow them to associate their guest session with a real user login. Kind of like when you start creating a shopping cart on an e-commerce site, then you login and your cart is copied to your user session.

Maybe as part of #32 we should be showing a warning on every page that the user is a guest and we could put the link to associate the session in that warning text.