Allow applications to have multiple "help" menu items

Summary:
Ref T7199. Convert the single help menu item into a dropdown and allow applications to list multiple items there.

When an application has mail command objects, link them in the menu.

Test Plan:
{F355925}

{F355926}

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T7199

Differential Revision: https://secure.phabricator.com/D12244

src/applications/diffusion/application/PhabricatorDiffusionApplication.php

@@ -22,8 +22,13 @@ final class PhabricatorDiffusionApplication extends PhabricatorApplication {
     return true;
   }
 
-  public function getHelpURI() {
-    return PhabricatorEnv::getDoclink('Diffusion User Guide');
+  public function getHelpDocumentationArticles(PhabricatorUser $viewer) {
+    return array(
+      array(
+        'name' => pht('Diffusion User Guide'),
+        'href' => PhabricatorEnv::getDoclink('Diffusion User Guide'),
+      ),
+    );
   }
 
   public function getFactObjectsForAnalysis() {