[web] [feat] [ENG-972] update modal header to match design

Summary:
https://linear.app/comm/issue/ENG-972/update-modal-header-to-match-design
https://www.figma.com/file/a1nkbWgbgjRlrOY9LVurTz/Comm-%2F-Desktop-app?node-id=1060%3A60538

Obviously, these don't look like the design (the entire modal). Just look at the header, and optional icon.

Test Plan: open modals, make sure header matches figma linked in linear task

Reviewers: atul, def-au1t, palys-swm, yayabosh

Reviewed By: palys-swm

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

Differential Revision: https://phabricator.ashoat.com/D3618