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

db/migrate/20200412214304_add_token_to_attachments.rb

@@ -0,0 +1,6 @@
+class AddTokenToAttachments < ActiveRecord::Migration[6.0]
+  def change
+    add_column :active_storage_attachments, :token, :string
+    add_index :active_storage_attachments, :token, unique: true
+  end
+end