Newsletter: send confirmation email using CiviCRM

From various discussions, we are ready to do the CiviCRM integration with the donate-neo email subscription form, where the process is:

• someone (user) enters their email in a public form
• donate-neo enqueues a message in the Redis queue, with the event type, their email and validation token
• CiviCRM reads this request, and sends a confirmation email using a message-template
• user receives the email and clicks the validation link, which is in donate-neo
• donation-neo enqueues another message in the Redis queue, to say that it's been validated

To test, we can use: https://staging.donate-review.torproject.net/subscribe/ (tor-www / blank)

cc @stephen hopefully I got that right?