Hide the "hidden" fields on custom form previews.
Before this change, Phabricator's custom form preview doesn't actually hide the
hidden fields, instead it shows them at 50% opacity. It looks cluttered and
doesn't provide a very useful "preview" at all.
This just hides some fields via CSS.
Cherry picked from:
https://phabricator.wikimedia.org/rPHABbba62cf5243538af9e37cc1211a01d247294f9f7
Upstream Task: