Internalize Maniphest task status colors

Summary: Ref T1812. These are a bit fluff and don't make too much sense to make configurable, at least for now.

Test Plan: Grepped for external callers.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T1812

Differential Revision: https://secure.phabricator.com/D8581