Add a contextual "scope" dropdown for searches

Summary: Add a "Search for ... in (document group)" thing that picks the current
scope based on the current application.

Test Plan: Conducted searches in several browsers.

Reviewers: btrahan, skrul

Reviewed By: btrahan

CC: aran, epriestley

Maniphest Tasks: T858

Differential Revision: https://secure.phabricator.com/D1610

src/applications/diffusion/controller/base/DiffusionController.php

@@ -51,6 +51,7 @@ abstract class DiffusionController extends PhabricatorController {
         ),
       ),
       null);
+    $page->setSearchDefaultScope(PhabricatorSearchScope::SCOPE_COMMITS);
 
     $page->appendChild($view);