Structure 'action' template as a ConfirmAction.

Markup the action transactional template so as to be actionable:

- an `EmailMessage` (http://schema.org/EmailMessage)
- containing a `HttpActionHandler` (https://developers.google.com/schemas/reference/types/HttpActionHandler)
- with a `ConfirmAction` (http://schema.org/ConfirmAction)

This gives the template direct actionability in the
Gmail Actions in the Inbox platform (https://developers.google.com/gmail/actions/), exposing
`ConfirmAction` (https://developers.google.com/gmail/actions/reference/one-click-action#confirm_action).