Always screen-off after a short amount of time

It looks like we have a pretty long screen-off time on the lockscreen; I think GNOME (and others like Android, if you want to count them) has a pretty short screen-off time on the greeter, which makes sense to me: there's not really much to monitor on the greeter, and we might as well save energy and turn off the screen when the user isn't interacting.

Something like 30 secs or a minute seem sane to me.

Note that this is separate from suspend, where we want to follow the user's settings (i.e. in case they have it set to never due to the computer being something like an always-on network hub).