Automate email notifications

[guusdk]

Emails are currently sent non-automatedly. There is tooling to generate the emails and to submit them to an SMTP server, but there's nothing which calls that automatically, and it is inherently stateful (tricky to get right in CI jobs; @horazont used to have a thing for that on GitLab using artifacts and caches, but it was fragile).

[Echolon]

Thank you Georg for the support!

• Estimated workload: 30 h
• Assumption: Knowledge of Github Automation, only de-duplication
• [ ] Further clarification: Does it require a VM or storage where the current state will be placed?

[horazont]

Closed in favour of #1236 and #1241, which should make for more clearer work packages. Re-open if you disagree.