Make the mobile menu available in "/mail/"

Summary: Ref T13244. See <https://discourse.phabricator-community.org/t/left-hand-menu-not-responsive-on-mobile/2358>.

Test Plan: {F6184160}

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13244

Differential Revision: https://secure.phabricator.com/D20093

src/applications/metamta/controller/PhabricatorMetaMTAMailListController.php

@@ -27,4 +27,8 @@ final class PhabricatorMetaMTAMailListController
     return $nav;
   }
 
+  public function buildApplicationMenu() {
+    return $this->buildSideNav()->getMenu();
+  }
+
 }