Show userId in expanded contact view

Show userId in expanded contact view.  Addresses issues raised in https://github.com/uProxy/uproxy/issues/412

For Google, this userId is often a @gmail.com address, but is sometimes a @public.talk.google.com address (I believe this is based on whether the users find each other initially through email, chat, or G+). For Facebook, we currently have a unique numeric userId, but will look for something more human-readable, like a @facebook.com email address or the unique URL of the contact.