Add a dropdown menu for Differential view options

Summary:
There are several open Differential tasks that are basically blocked on not
having reasonable places in the UI to put things. Replace the "View Standalone /
Raw" button with a "View Options" dropdown menu so we can shove things like
"Expand All", "Fold / Unfold File", and "View in Diffusion" in there.

This doesn't change any behavior, just puts the existing options in a menu.

Test Plan:
  - Toggled menu open by clicking button.
  - Clicked menu items.
  - Toggled menu closed by clicking button.
  - Toggled menu closed by clicking document.
  - Toggled menu closed by opening another menu.
  - Toggled menu closed by selecting an item.

Reviewers: btrahan, jungejason

Reviewed By: btrahan

CC: aran, btrahan

Maniphest Tasks: T497, T309

Differential Revision: https://secure.phabricator.com/D1316

src/applications/differential/controller/changesetview/DifferentialChangesetViewController.php

@@ -194,25 +194,6 @@ class DifferentialChangesetViewController extends DifferentialController {
     $detail->setChangeset($changeset);
     $detail->appendChild($output);
 
-    if (!$vs) {
-      $detail->addButton(
-        phutil_render_tag(
-          'a',
-          array(
-            'href' => $request->getRequestURI()->alter('view', 'old'),
-            'class' => 'grey button small',
-          ),
-          'View Raw File (Old Version)'));
-      $detail->addButton(
-        phutil_render_tag(
-          'a',
-          array(
-            'href' => $request->getRequestURI()->alter('view', 'new'),
-            'class' => 'grey button small',
-          ),
-          'View Raw File (New Version)'));
-    }
-
     $detail->setRevisionID($request->getInt('revision_id'));
 
     $output =