Add email hooks for mailgun callback event.
Add mail service.
Add email subscriptions service.
Unsubscribe from database when mailgun callback is sent.
Add to mailing list when user register and need email notification.
Send mail to mailing list by cron.
Email notification by schedule.
Merge branch 'feature/email_notification' into develop
Merge branch 'develop' of github.com:opendream/sicksense into develop