[web] Increase clickable region of ChatThreadListSidebar

Summary:
Basically D3543 but for ChatThreadListSidebar

Clickable area before:

Clickable area after:

Test Plan: Looks and behaves as expected: [will drag in video on phabricator]

Reviewers: ashoat, palys-swm, def-au1t

Reviewed By: ashoat, palys-swm

Subscribers: Adrian, karol-bisztyga, benschac, yayabosh

Differential Revision: https://phabricator.ashoat.com/D3545