Adjust reviewer transaction icon in Differential

Summary:
Ref T4866. I did a fancy version of this but it looks pretty bad/confusing so here's a simple version.

Fancy-but-whack version:

{F146847}

Test Plan: This version is like that, but just always uses fa-user.

Reviewers: chad

Reviewed By: chad

Subscribers: epriestley

Maniphest Tasks: T4866

Differential Revision: https://secure.phabricator.com/D8840