[web] Fix sidebar hover styling in ChatThreadListSidebar

Summary: Sidebars in the thread list were not being highlighted on hover.. figured they should be.

Test Plan:
Before:

After:

Reviewers: palys-swm, def-au1t, benschac, varun

Reviewed By: palys-swm

Subscribers: ashoat, Adrian, karol-bisztyga

Differential Revision: https://phabricator.ashoat.com/D3361