[web] Style ChatThreadListItemMenu vertical ellipsis on hover

Summary: To address feedback from @palys-swm here: https://phabricator.ashoat.com/D3543#97051

Test Plan: Looks as expected:

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

Reviewed By: ashoat

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

Differential Revision: https://phabricator.ashoat.com/D3575