…lesheets.

And remove some of the ::placeholder and ::cue hacks where we need to use
!important to make the property not apply for content but apply on UA sheets.

The comment about the white-space property was wrong, we don't enforce it with
!important in the UA stylesheets for <input> (we do for <textarea> though), so
I've kept the flag since it really applies.

Differential Revision: https://phabricator.services.mozilla.com/D37717