R1:7713fb5d99a3

Add config validation for task status config

Summary: Ref T1812. This still doesn't expose configuration to the user, but adds validation for it.

Test Plan: Added a pile of unit tests.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T1812

Differential Revision: https://secure.phabricator.com/D8584