Herald - make tokenizers have the purdy icons

Summary: ...use the prefab stuff as it does fancier things than we were doing. Only trick then really is to pass username and the map of handle phids => icons to the client so prefab can work nicely. Fixes T4775.

Test Plan: made a herald rule with projects and users. Saw nice icons. Reloaded page and still saw nice icons.

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: epriestley, Korvin

Maniphest Tasks: T4775

Differential Revision: https://secure.phabricator.com/D8749