Improve feed rendering of user rename story

Summary:
Ref T13244. This story publishes to the feed (and I think that's reasonable and desirable) but doesn't render as nicely as it could.

Improve the rendering.

(See T9233 for some context on why we render stories like this one in this way.)

Test Plan:

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13244

Differential Revision: https://secure.phabricator.com/D20097