Profile Pictures can only be edited for System Agents

Summary:
I feel pretty disturbed by having the thought of admins,
super-beings among all users, being able to edit my profile picture. So I took away
that capability of theirs.

Test Plan: Tried to edit a sys agent's profile pic, and somebody else's.

Reviewers: epriestley, chad, btrahan, Afaque_Hussain

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D5358