Display an invitation notice when user is alone

As @fqueze [noticed](https://github.com/mozilla/talkilla/pull/11/files#r3554721):

> So the "As the first user logged in, I see a message that I am the only person logged in, and see an offer to invite friends" story is no longer done, then?

So here's a patch for fixing this issue.

r=@fqueze