Add Phrequent workflows to Arcanist

Summary:
Depends on D9906.

This adds `arc start`, `arc stop` and `arc tracking` for tracking tasks, diffs and other objects in Phrequent.

Test Plan: Tested this against a local install.

Reviewers: skyronic, #blessed_reviewers, epriestley

Reviewed By: #blessed_reviewers, epriestley

Subscribers: maxhodak, hach-que, aran, epriestley, Korvin

Maniphest Tasks: T3569, T3969

Differential Revision: https://secure.phabricator.com/D7327