Streamline email functionality

**Is your feature request related to a problem? Please describe.**
Email is sent in two places: in `notifications.py` and in `util.py` (`send_email` function).
Via #416 some fixes were made in `notifications.py` to cope with various email configurations TLS, no Auth etc, for both Docker and regular installs. This is for Resource notification only. All other email (lost password, status summary, ..) is still sent
with `util.py-send_email`.

**Describe the solution you'd like**
Provide a single `send_email()` function for all email-sending cases. Best is to take `util.py-send_email` and extend with the implementation in `notifications.py`. Make the function robust for config errors. May still be lean/silently fail, on `login()` failure as that is optional and may break existing installs.

**Describe alternatives you've considered**
We could discard `SMTPLib` and use a higher level email lib. But that also introduces an additional dependency, plus may break existing installations.

**Additional context**
N.A.

Lies die Funktion send_email in util.py und die E-Mail-Implementierung in notifications.py und verfolge anschließend ihre Aufrufer für Ressourcenbenachrichtigungen, E-Mails für verlorene Passwörter und Statuszusammenfassungen. Vergleiche das im Issue beschriebene Konfigurations- und Fehlerverhalten; abgeschlossen bedeutet, dass alle Fälle des E-Mail-Versands eine einzige robuste send_email-Funktion verwenden, ohne das optionale Login-Verhalten zu beeinträchtigen.

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