Add demo page

The demo page shows a form and an embedded google sheet side by side.
The form needs be be configured with a DEMO_FORM_ID environment variable.

config/initializers/demo.rb

@@ -0,0 +1 @@
+DEMO_FORM = Form.find_by(id: ENV['DEMO_FORM_ID'])

config/routes.rb

@@ -13,9 +13,11 @@ Rails.application.routes.draw do
   # short URL for form page
   get '/s/:id/form' => 'forms#form', as: :form_submitter
 
+  get '/signup' => 'sessions#new', as: :signup # TODO: add proper signup page
   get '/login' => 'sessions#new', as: :login
   get '/logout' => 'sessions#destroy', as: :logout
   get '/auth' => 'sessions#auth', as: :auth
 
+  get '/demo' => 'home#demo', as: :demo
   root 'home#index'
 end