Modal operation

I think this should be enough work to close #19 (I'd have liked to attach this pull-request to that issue, but apparently that's not a thing?). 

Anyway, notable points are that if we do nothing, the app will always be in cfp mode.  We can set a new mode by setting (or changing) the ENV['VESTIBULE_MODE'] value.  As far as I can tell Heroku will restart the app when we change any ENV values.

Any thoughts or should I just merge / release?