[web] [feat] add mark as unread styles for selected and hovered states

Summary:
per conversation in D3422 updating active and hovered styles for mark as unread (this is option 1)

Test Plan: hover and select chat thread list items, click mark as unread menu button. Make sure they match.

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

Reviewed By: ashoat

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

Differential Revision: https://phabricator.ashoat.com/D3499