Enabling Admins to set profile pictures for System Agents

Summary: Added an action in Edit option of People application allowing Admins to set profile pictures for System Agents

Test Plan: By trying to set a profile picture for the sytem agents

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D5293