R1:66b74073be68

Provide ANSI color information for Harbormaster build status via API

Summary:
Ref PHI261. This moves Harbormaster build status to work more similarly to other modern status types, like Differential revision status, where we try to specify as much behavior on the server as possible so that the client and server can vary independently.

(I don't have any specific plans to make Harbormaster build status configurable on the server side, but it isn't out of the question.)

Test Plan: Ran `harbormaster.