R1:f54dfe756769

Add a basic icon typeahead

Summary: Fixes T11971, adds a basic typeahead for selecting an icon.

Test Plan:
http://local.phacility.com/typeahead/browse/PhabricatorIconDatasource/

{F2561013}

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin

Maniphest Tasks: T11971

Differential Revision: https://secure.phabricator.com/D17301