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.

Leggi la funzione send_email di util.py e l’implementazione delle email in notifications.py, quindi traccia i relativi chiamanti per le notifiche delle risorse, le email per la password dimenticata e i riepiloghi dello stato. Confronta la configurazione e il comportamento in caso di errore descritti nell’issue; il lavoro è completo quando tutti i casi di invio delle email usano un’unica funzione send_email robusta senza compromettere il comportamento di accesso opzionale.

Scritto dal modello di indicizzazione a partire dal testo della issue.