[web] [refactor] css thread tab css logic

Summary:
make css and react classnames a bit simpler. We didn't need the additional active and inactive classes and true css in the classname definition.

Test Plan: Toggle chat tabs, functionality should remain the same

Reviewers: atul, ashoat

Reviewed By: atul, ashoat

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

Differential Revision: https://phabricator.ashoat.com/D3318