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);
 

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

@@ -14,6 +14,7 @@ phutil_require_module('phabricator', 'applications/diffusion/query/pathid/base')
 phutil_require_module('phabricator', 'applications/diffusion/request/base');
 phutil_require_module('phabricator', 'applications/diffusion/view/base');
 phutil_require_module('phabricator', 'applications/phid/handle/data');
+phutil_require_module('phabricator', 'applications/search/constants/scope');
 phutil_require_module('phabricator', 'infrastructure/env');
 phutil_require_module('phabricator', 'view/layout/crumbs');
 phutil_require_module('phabricator', 'view/layout/panel');