Add mailpile to the mail group

src/freedombone-app-mailpile

     adduser --system --home=/var/www/$MAILPILE_DOMAIN_NAME/mail/ --group mailpile
     adduser mailpile debian-tor
     adduser mailpile www-data
+    adduser mailpile mail
     adduser mailpile $MY_USERNAME
     chown -R mailpile: /var/www/$MAILPILE_DOMAIN_NAME/mail/