[web] [feat] [ENG-716] add icon to modal header
Summary:
add icon to modal header, still not styled, just adding the icon per the design
https://www.figma.com/file/a1nkbWgbgjRlrOY9LVurTz/Comm-%2F-Desktop-app?node-id=1060%3A60514
https://linear.app/comm/issue/ENG-716/introduce-leave-channel-modal
delete and leave channel both call for an icon
Test Plan: modal should render with icon when leaving thread
Reviewers: atul, def-au1t, palys-swm, ashoat
Reviewed By: atul, def-au1t, palys-swm, ashoat
Subscribers: ashoat, palys-swm, Adrian, karol-bisztyga, yayabosh
Differential Revision: https://phabricator.ashoat.com/D3615