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

src/applications/differential/controller/DifferentialRevisionViewController.php

@@ -156,7 +156,7 @@ final class DifferentialRevisionViewController extends DifferentialController {
         phutil_tag(
           'a',
           array(
-            'class' => 'button grey',
+            'class' => 'button button-grey',
             'href' => $request_uri
               ->alter('large', 'true')
               ->setFragment('toc'),