Allow administrators to edit System Agent information from the agent's profile

Summary:
Ref T4065. Currently, we have this super copy/pasted "edit profile picture" UI for system agents.

Instead, give administrators direct access from profiles, so they can use the same code pages do.

Test Plan: Edited my profile picture and profile details. Edited an agent's. Was unable to edit a non-agent user.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T4065

Differential Revision: https://secure.phabricator.com/D8664