Fix PhabricatorActionView::getAvailableIcons
Summary: getAvailableIcons parses icon(s).json, which has recently been pluralized in D5890.
Test Plan: /uiexample/view/PhabricatorActionListExample/
Reviewers: epriestley
Reviewed By: epriestley
CC: aran, Korvin
Differential Revision: https://secure.phabricator.com/D5902