E-Mail notifications

https://www.gitmemory.com/issue/bigbluebutton/greenlight/878/589527655

Example settings in the SMTP section of .env file:

ALLOW_MAIL_NOTIFICATIONS=true 
GREENLIGHT_DOMAIN=ourdomain.com 
GREENLIGHT_USE_WEBHOOKS=false
SMTP_SERVER=smtp.ourdomain.com 
SMTP_PORT=465 
SMTP_DOMAIN=ourdomain.com 
SMTP_USERNAME=systemsmtp@ourdomain.com 
SMTP_PASSWORD=systemsmtppassword 
SMTP_AUTH=plain (also tested ssl or tls) 
SMTP_STARTTLS_AUTO=true SMTP_SENDER=bbb@ourdomain.com