implement an onboarding flow

We redid the unauthenticated homepage in #2544, dropping back to a very simple call to action. In the process we lost what little meaningful onboarding of new users we had in the form of the leaderboards. In #2584 we've started talking about building out a homepage for authenticated users, and I believe that onboarding is a vital part of that. I'm thinking of the signed-in homepage as a dashboard that focuses on the four interesting things you can do on Gittip:

 - Give money
 - Receive money
 - Take money (from Teams)
 - Commune (though #967 )

I'm thinking we want an onboarding workflow that involves all four of these elements, prioritized in that order.