Create metamta.user-address-format configuration option

Summary: this lets users specify what "name" to use in email addresses

Test Plan: changed the conf setting in my local instance and used phabricator. observed name format changes

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Maniphest Tasks: T1862

Differential Revision: https://secure.phabricator.com/D3639