[web] [fix] chat tab icon alignment and spacing

Summary:
Align the chat tabs per the design. After re-sizing our icons, some of our icon layout css needed to be updated/fixed to match our design.

Test Plan:
Make sure chat tabs visually match the design.

https://www.figma.com/file/a1nkbWgbgjRlrOY9LVurTz/?node-id=1170%3A77104

Reviewers: atul, ashoat

Reviewed By: atul, ashoat

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

Differential Revision: https://phabricator.ashoat.com/D3316