Introduce HarbormasterBuildTarget to snapshot build steps through a build

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