[web] [refactor] thread-list-sidebar, active thread style to use classnames lib instead of ternary

Summary: use the classnames library rather than ternary.

Test Plan: Make sure active class still renders.

Reviewers: atul, ashoat

Reviewed By: ashoat

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

Differential Revision: https://phabricator.ashoat.com/D3126