send email notifications

[fembau]

This issue is about a notification module, the basic functionally and requirements to be able to send email notifications (p.ex. for #893).

TODOs:

• create infrastructure concept for sending emails:
  • make sending available for which RSD components? backend, scraper, ...
  • how sending emails is done?
    • p.ex. directly via SMTP, new API, use postfix as relay host, ...
    • own e-mail server vs. relay host
    • requirements for using a relay host
    • ...
  • where the notification module lives? own container, ...
  • how the notification module is configured?
  • how the notification module is deployed?
• clarify legal, privacy and data protection aspects:
  • For which use cases we are allowed to use the email-address and to send emails
  • update the privacy terms
  • update the registration process?

Links:

• http://www.postfix.org
• https://docs.nginx.com/nginx/admin-guide/mail-proxy/mail-proxy