[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

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

Differential Revision: https://phabricator.ashoat.com/D3542