Modernize "arcanist project" datasource

Summary: Ref T4420. Do arc projects.

Test Plan:
  - Used Herald typeahead.
  - Used Repositories typehaead.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T4420

Differential Revision: https://secure.phabricator.com/D9879