Implement showing buildable status in Diffusion

Summary: This implements showing the buildable status in Diffusion and unifies some of the logic used to calculate and render build and buildable statuses.

Test Plan: Looked at diffs and commits with statuses, they rendered fine. Looked at Diffusion and saw buildable status appear (with a manual buildable and manual buildables included in the query).

Reviewers: #blessed_reviewers, epriestley, chad

Reviewed By: #blessed_reviewers, epriestley

Subscribers: epriestley, Korvin

Differential Revision: https://secure.phabricator.com/D9496