Consolidate outbound mail status in a new class
Summary: Ref T5791. This collects outbound mail status in one place and makes the list view a little spiffier.
Test Plan: Looked at list and detail views. Grepped for changed classes/constants.
Reviewers: chad
Reviewed By: chad
Maniphest Tasks: T5791
Differential Revision: https://secure.phabricator.com/D13884