Use same method to get object URI as used in DifferentialTransactionEditor and PhabricatorApplicationTransactionEditor

Summary: Maniphest object has `getURI` method, let's use it

Test Plan: Create event in task - URI generated as expected in email notification

Reviewers: epriestley, Pawka, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: Korvin

Differential Revision: https://secure.phabricator.com/D20935

src/applications/maniphest/editor/ManiphestTransactionEditor.php

@@ -226,7 +226,7 @@ final class ManiphestTransactionEditor
 
     $body->addLinkSection(
       pht('TASK DETAIL'),
-      PhabricatorEnv::getProductionURI('/T'.$object->getID()));
+      $this->getObjectLinkButtonURIForMail($object));
 
 
     $board_phids = array();