Move user approval to modular transactions

Summary: See https://discourse.phabricator-community.org/t/how-to-approve-user-via-conduit-api/2189. This particular use case doesn't seem very compelling, but moving this logic out of PhabricatorUserEditor is a win anyway.

Test Plan: Registered a new user, approved/unapproved them conduit, approved from the UI.

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin

Differential Revision: https://secure.phabricator.com/D19877