Move user renames to modular transactions

Summary: Cleaning up more super-old code from PhabricatorUserEditor. Also fix user logging in approve transactions. I'm not sure how it worked at all previously.

Test Plan: Created new users, renamed them, checked DB for sanity. Entered invalid names, duplicate names, and empty names, got appropriate error messages.

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin

Differential Revision: https://secure.phabricator.com/D19887