CA-925 Update EmailSubscription model to store email address instead of user id
Get unsubscribed emails from MailChimp
CA-927 Check if email was unsubscribed before sending email
CA-928 Base unsubscribe actions off of email, not user
CA-929 Add support to unsubscribe from a list on MailChimp via their API
CA-188 Support ability for some emails to be unsubscribable and ignore subscription