Make typeahead datasources default to PHID type icons

Summary:
Ref T4420. If a datasource does not specify an icon explicitly, check if the PHID type has a default, and use that.

This leaves us with only Projects and some special stuff setting explicit icons, and reduces code duplication.

Test Plan: Used typeahead to find all affected object types.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T4420

Differential Revision: https://secure.phabricator.com/D9894