Modernize "user, project or package" typeahead datasource

Summary: Ref T4420. Call this "auditor" since that's what it is.

Test Plan:

• Edited auditors in auditor search.
• Edited auditors in "add auditors" in Diffusion.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T4420

Differential Revision: https://secure.phabricator.com/D9888