[web] [refactor] simplify username logic in message preview

Summary: simplify rendering logic for username in message preview

Test Plan: username logic is the same as production. No new functionality

Reviewers: atul, def-au1t, palys-swm, ashoat

Reviewed By: ashoat

Subscribers: ashoat, palys-swm, Adrian, karol-bisztyga

Differential Revision: https://phabricator.ashoat.com/D3506