[#54 ] Sign outgoing mails (SMIME)

Closes #54 (closed)

• Enable SMIME by using smimesmtp backend, and set 2 files for certificate

EMAIL_BACKEND=vendor.django_mail.backends.smimesmtp.EmailBackend
EMAIL_SMIME_CERT_FILE_PATH=/etc/notifications-noreply.pem
EMAIL_SMIME_CERT_KEY_FILE_PATH=/etc/notifications-noreply-key.pem

should be used for mail / mail_daily / mail_weekly / mail_monthly

Requires https://pypi.org/project/python-smail/ and probably asn1 module upgrade.