hide the honeypot field, and fix $muted reference as $form-muted

If you enable the anti-spam honeypot in a form's settings, it needs to be hidden. I've just picked up Gravity Forms' rules and used them here, but of course without the excessive selectors or `!important` since the target is a sane custom theme :)

I also noticed that there's a reference to `$muted` which I assumed was meant to be `$form-muted`, so I've changed that too. I didn't do anything with `$white` or `$gray` as I assume we're meant to define those in our own stylesheets.