[web] [fix] ancestorThreads styling
Summary:
fix up styles. Right size chevrons, align correctly
https://www.figma.com/file/a1nkbWgbgjRlrOY9LVurTz/Comm-%2F-Desktop-app?node-id=1170%3A77116
Assigning @ashoat too since Atul is OOO.
Test Plan: n/a make sure it's close to figma
Reviewers: atul, ashoat
Reviewed By: ashoat
Subscribers: ashoat, palys-swm, Adrian, karol-bisztyga
Differential Revision: https://phabricator.ashoat.com/D3183