Modernize "user or project" typeahead datasources

Summary: Ref T4420. These are used for some stuff like "reviewer".

Test Plan:

• Edited "reviewers" in differential edit.
• Edited "reviewers" in differential search.
• Edited "reviewers" in Differential "add reviewers..." action on detail page.
• Edited a "reviewers" field in a herald rule.
• Edited "owner" in owners search.
• Edited "primary owner", "owners" on owners edit.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T4420

Differential Revision: https://secure.phabricator.com/D9887