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, ar, vishal

Subscribers: mattr, bryan, jkao

Differential Revision: https://tails.corp.dropbox.com/D60301
Update module cache