Add a "Red/Green Colorblind" accessibility mode, make all web UIs and email respect it

Summary:
Fixes T12172. Fixes T12060. This allows runtime code building CSS for mail to read CSS variables, then makes all the code do that.

It reverts the non-colorblind red/green to the colors in use before T12060, which seem better for non-colorblind users since no one really complained?

Test Plan:

• Viewed code diffs in Web UI.
• Viewed prose diffs in Web UI.
• Viewed code diffs in email.
• Viewed prose diffs in email.

All modes respected the accessibility color scheme.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T12172, T12060

Differential Revision: https://secure.phabricator.com/D17269