Summary: This implements build targets as outlined in D7582. Build targets represent an instance of a build step particular to the build. Logs and artifacts have been adjusted to attach to build targets instead of build / build step pairs.
Test Plan: Ran builds and clicked around the interface. Everything seemed to work.
Reviewers: epriestley, #blessed_reviewers
Reviewed By: epriestley
CC: Korvin, epriestley, aran
Maniphest Tasks: T4111, T1049
Differential Revision: https://secure.phabricator.com/D7703
