Update accountadmin to use new admin empowerment code

Summary: Fixes https://discourse.phabricator-community.org/t/admin-account-creation-fails-call-to-undefined-method-phabricatorusereditor-makeadminuser/2227. This callsite got skipped when updating the EmpowerController to use the new transactional admin approval code.

Test Plan: Invoked accountadmin to promote a user, no longer got an exception.

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin

Differential Revision: https://secure.phabricator.com/D19915