Follow-up: Style form_input/textarea readonly and disabled states following Pajamas specs

The purpose of this follow-up issue is styling the form_input/textarea readonly/disabled states following Pajamas specs.

The following discussion from !1341 (merged) should be addressed:

• @tauriedavis started a discussion: (+1 comment)

  Looks great! I think this is ready to merge.

  The invalid readonly/disabled textarea is incorrect, is there a separate issue to fix that or should we create one?

  Also I'm not sure about the two way bound input. Do we use that anywhere? Can it be removed or do we need to update the text underneath to use the current font/spacing? This seems separate from this MR so happy to open a separate issue to take a look.