Fixing non-square profile images for hovercards

Summary: Refs T1048 - disables bg repeat, centers profile image

Test Plan: looked at my own hovercard in my local install, which has a non-square prof pic

Reviewers: epriestley, chad

Reviewed By: epriestley

CC: aran, Korvin

Maniphest Tasks: T1048

Differential Revision: https://secure.phabricator.com/D5604