Add config to require real name, respect config when creating new users, drop real name from full name if not provided.

Summary: Fixes T4728, first pass, Make real name optional on user accounts

Test Plan: Default real name config should be false (not required). Create new user, real name should not be required. Toggle config, real name should be required. Users with no real name should be always listed by their usernames.

Reviewers: Blessed Reviewers, epriestley

Reviewed By: Blessed Reviewers, epriestley

Subscribers: epriestley, Korvin

Maniphest Tasks: T4728

Differential Revision: https://secure.phabricator.com/D9027