Fix tour breaking when no scores <div> is present

Closes #424
Show the tour dialog whenever the guest ID changes

Ref #425
Transparent background on money and score icons
Optimise modern scene images

50-60% saving in many cases!
Redirect root page visits to the scenario
Add GuestController#set_guest

Permits a controller to specify a fully-formed Guest object to use when
creating the session.
Add SceneController#survey

Given a token from an external website, sets up the guest session and
directs the user to a new scenario. If a visitor has already created a
scenario with the token, they are probably a return visitor, and we
redirect them to that scenario.