R1:821df3364ec4

Hide the "hidden" fields on custom form previews

Summary:
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:

https://phabricator.wikimedia.