copy_notification_image.sh: allow force
copy_notification_image.sh: allow force

* If the passed-in EXPIRES_IN_SECONDS is 0, then force sending the notification, even if we just sent a different one.
This will be used by notifications that typically stay for a while, like taking dark frames, non-recoverable error, and not taking pictures during daytime
thomasjacquin/allsky,thomasjacquin/allsky,thomasjacquin/allsky