Add "Relay" button to relay email to another SMTP server

Also adds command-line options:

    --outgoing-host <host>  SMTP host for outgoing emails
    --outgoing-port <port>  SMTP port for outgoing emails
    --outgoing-user <user>  SMTP user for outgoing emails
    --outgoing-pass <pass>  SMTP password for outgoing emails
    --outgoing-secure       Use SMTP SSL for outgoing emails