Move integration to Herald + Callsigns

Summary:
This shifts away from Harbormaster due to some feature limitations currently (both internally at Dropbox and within Phabricator).

Additionally it moves towards the more recent phabricator.callsign repository option which will give a stronger guarantee that builds can be created.

Reviewers: cf, vishal, ar

Reviewed By: ar

Subscribers: mattr, bryan, jkao

Differential Revision: https://tails.corp.dropbox.com/D60301