Modernize "legalpad" typeahead datasource

Summary: Ref T4420. Modernize legalpad.

Test Plan:

• Used typeahead in Herald rules.
• Used typeahead in Policy dialog.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T4420

Differential Revision: https://secure.phabricator.com/D9876