Use herald to trigger builds of revisions and commits.

Summary:
Depends on D7500.

This seemed like a pretty good idea once I thought of it.  Instead of having some custom triggering logic instead Harbormaster, I figured it best to leverage all of Herald's power so that users can create rules to apply builds to commits and differential revisions.  This gives the added advantage that they can trigger off builds for particular types of revisions and commits, which seems like it could be really useful (e.g. run extra tests against revisions that touch sensitive areas of the code).

Test Plan: Ran the usual daemons + the Harbormaster daemon.  Pushed a commit to the repository and saw both the buildable and build get created when the commit worked picked it up.  Submitted a diff and saw both the buildable and build get created when the Herald rules were evaluated for the diff.

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley

CC: Korvin, epriestley, aran, hwinkel

Maniphest Tasks: T1049

Differential Revision: https://secure.phabricator.com/D7501