Separate button CSS classes

Summary: Try to dis-ambiguate various button types and colors. Moves simple to phui-button-simple and moves colors to button-color.

Test Plan: Grep for buttons still inline, UIExamples, PHUIX, Herald, and Email Preferences.

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin

Differential Revision: https://secure.phabricator.com/D18077