[web] [refactor] message preview styles solidified into one ternary statement

Summary: move styles into a single call

Test Plan: click and hover about chat thread list items. Functionality should be the same

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

Reviewed By: palys-swm, ashoat

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

Differential Revision: https://phabricator.ashoat.com/D3505