[web] [fix] arrows touch per IRL chat with ashoat

Summary:
had to toggle menu button size to make arrows touch. Additionally, there wasn't any design for sub thread unread menu on hover.

Test Plan: make sure it looks like the screen shot

Reviewers: atul, ashoat

Reviewed By: ashoat

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

Differential Revision: https://phabricator.ashoat.com/D3165