Provide a basic search engine for builds

Summary:
This supports a few basic use cases that aren't served by the buildable search engine:

• I'm trying to discover when the last time that this particular build plan failed was.
• I want to know if any builds have deadlocked.
• At a glance, I'm more interested in what build plans are running, not which buildables are being built. This is more often than not the case.

Test Plan: {F1744003}

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: Korvin, epriestley

Differential Revision: https://secure.phabricator.com/D16347