[Redesign] Full High Contrast Mode
Summary: Ref T8099, Ref T8614. Adds a more complete full contrast mode.
Test Plan: Bop through UIExamples, check tokens, diffs, tasks, workboards, buttons, info views, looks sharp.
Reviewers: btrahan, epriestley
Reviewed By: epriestley
Subscribers: epriestley, Korvin
Maniphest Tasks: T8099, T8614
Differential Revision: https://secure.phabricator.com/D13375