[web] Refactor classNames in threadRow to take object instead of array
Summary: Pass object instead of array into classNames to make code more readable. Cut ternary operator and made everything more symmetric.
Test Plan: Ensured that output was the same before and after the change in Chrome/Safari.
Reviewers: atul, ashoat
Reviewed By: atul, ashoat
Subscribers: ashoat, palys-swm, Adrian, karol-bisztyga, benschac
Differential Revision: https://phabricator.ashoat.com/D3542