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/models/form.rb

@@ -13,11 +13,11 @@ class Form < ApplicationRecord
 
   # TODO: use counter_cache option on association
   def submissions_count
-    submissions.count
+    @submissions_count ||= submissions.count
   end
 
   def last_submission_date
-    submissions.order(created_at: :desc).first&.created_at
+    @last_submission_date ||= submissions.order(created_at: :desc).first&.created_at
   end
 
   def deactivate!(reason = nil)