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