Mostly modernize the policy control typeahead

Summary:
Ref T4420. This doesn't share all the code it really should, and renders a little odd. Make it more standard.

(Icons aren't handled totally correctly but there's no usability impact and all that code should just get cleaned up.)

Test Plan: Used custom policy typeahead, had a more standard experience.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T4420

Differential Revision: https://secure.phabricator.com/D9902