[web] [fix] ENG-730 add notif dot for thread

Summary:
adds notif dot to thread in conversation selector
https://linear.app/comm/issue/ENG-730/unread-indication-for-sidebars-is-very-strange

Test Plan: send your self a message from one user to another, click sidebar to toggle off unread.

Reviewers: atul, def-au1t, palys-swm

Reviewed By: atul

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

Differential Revision: https://phabricator.ashoat.com/D3444