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