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