[web] [fix] thread list size and paddings are incorrect

Summary:
fix padding sizes to reflect figma more closely https://www.figma.com/file/RROVwCKAeFaO7tG64blzlx/Comm-Web-Design-System?node-id=1%3A17

still not 100% but a lot closer. The rest of the visual issues will be solved in future diffs.

Test Plan: check figma, make sure screen shots are close.

Reviewers: atul, ashoat

Reviewed By: atul, ashoat

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

Differential Revision: https://phabricator.ashoat.com/D3480