Correctly localize times in the user list

Summary: We currently show a user's signup time in their local time, not the
viewer's local time. Oops!
Test Plan: Looked at user list.
Reviewed By: tuomaspelkonen
Reviewers: toulouse, jungejason, tuomaspelkonen, aran
CC: aran, epriestley, tuomaspelkonen
Differential Revision: 585