[web] Add ChatThreadListItemMenu to ChatThreadListSidebar

Summary: This adds mark-as-read/mark-as-unread functionality for sidebars on web. I also made some minor style changes for the menu that apply outside of sidebars (namely, decreased size).

Test Plan: Test mark-as-read and mark-as-unread functionality

Reviewers: KatPo, palys-swm

Reviewed By: KatPo, palys-swm

Subscribers: zrebcu411, Adrian

Differential Revision: https://phabricator.ashoat.com/D462