This change updates our main forms CSS definitions to use Ink-defined

colors and spacing throughout. This includes several vaguely

interrelated pieces:

Fixed up colors throughout, removing hard-coded color codes and

  #rb-ns-ui.colors variables in favor of Ink CSS variables.
Moved old LESS-based variables into clearly-denoted legacy sections.
Updated field help text to use an ink-provided icon and flexbox-based

  layout, fixing up some ugly spacing and alignment.
Fixed an issue where opening the inline editor for draft comment

  replies caused the text to jump horizontally (we apparently had a rule

  for the horizontal margins get mistakenly deleted?).
Fixed up some existing definitions of CSS variables to use correct

  type prefixes.

Went through a variety of forms in the admin UI and verified

  appearance of form fields and fieldsets (both expanded and collapsed).
Verified appearance and functionality of textarea/CodeMirror editors

  in review request fields, reviews, and replies.
Verified that opening the editor for all inline editable

  textarea/CodeMirror fields did not cause any visual jumps.
Verified the appearance of <input> and <textarea> elements in other

  parts of the UI, including review request fields and reviews.