Integrate Phabricator with new syncer

Created by: tsenart

• Integrate Phabricator with new syncer
  • Unified Phabricator client #3533
  • Implement PhabricatorSource and enable new syncer for Phabricator external services
  • Implement phabricator.exclude.
  • Update UI to remove enabled state for Phabricator repos.
  • Update UI to remove explicit deletion for Phabricator repos.

Follow up on #2025 (closed) Part of #3467 (closed)