[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