Make phrasing on "Mark All Read" consistent

Summary: Fixes T5025. We label this action in two different ways ("Clear All Notifications", "Mark All Read"). The latter is probably a better description.

Test Plan: looked at the UI.

Reviewers: btrahan, chad

Reviewed By: chad

Subscribers: epriestley

Maniphest Tasks: T5025

Differential Revision: https://secure.phabricator.com/D9073

src/applications/notification/controller/PhabricatorNotificationPanelController.php

@@ -42,7 +42,7 @@ final class PhabricatorNotificationPanelController
           'href' => '/notification/clear/',
           'class' => 'phabricator-notification-clear-all'
         ),
-        pht('Clear All Notifications')),
+        pht('Mark All Read')),
       $content,
       phutil_tag(
         'a',