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.

app/javascript/packs/application.js

@@ -9,6 +9,8 @@ require("@rails/activestorage").start()
 require("channels")
 
 require('burger_menu');
+require('tinyforms');
+require('demo');
 
 // Uncomment to copy all static images under ../images to the output folder and reference
 // them with the image_pack_tag helper in views (e.g <%= image_pack_tag 'rails.png' %>)