[web] [fix] [ENG-716] use cross icon in modal
Summary:
swap x span for SWM icon
https://www.figma.com/file/a1nkbWgbgjRlrOY9LVurTz/Comm-%2F-Desktop-app?node-id=1061%3A63258 -- some modals have the x button, others don't: https://www.figma.com/file/a1nkbWgbgjRlrOY9LVurTz/Comm-%2F-Desktop-app?node-id=1062%3A64128
x button is now optional
Test Plan: render icon
Reviewers: atul, palys-swm, def-au1t
Reviewed By: palys-swm
Subscribers: ashoat, palys-swm, Adrian, karol-bisztyga, yayabosh
Differential Revision: https://phabricator.ashoat.com/D3616