Use secure token to create attachment links

The link is public but protected by a secure token.

This extends the activesupport Attachment model to automatically
generate a token

app/models/submission.rb

@@ -37,7 +37,7 @@ class Submission < ApplicationRecord
       attachment = ActiveStorage::Attachment.new(record: self, name: 'files', blob: create_one.blob)
       attachment.save
       # return the URL that we use to show in the Spreadsheet
-      Rails.application.routes.url_helpers.file_upload_url(form_id: form, submission_id: self, id: attachment.id, host: DEFAULT_HOST)
+      Rails.application.routes.url_helpers.file_upload_url(form_id: form, submission_id: self, id: attachment.token, host: DEFAULT_HOST)
     else
       value.to_s
     end