Show user profile images on User list

Summary: Ref T4400. Same deal as projects. Tweaked the CSS a touch to make it look better in these views.

Test Plan: Viewed /people/.

Reviewers: chad, btrahan

Reviewed By: btrahan

Subscribers: epriestley, chad

Maniphest Tasks: T4400

Differential Revision: https://secure.phabricator.com/D8571