An interactive walkthrough for new users.

- Five-step tour on first launch, with a skip button on every screen
- The tour ends by sending a real burn note - encrypted on your device,
  real link, one view, five minutes - so you have used BurnPony before you
  have finished setting it up
- Replay the walkthrough any time from Settings

If you have already sent a note, the tour is not shown on upgrade.
