fix(modal): Make x to close react modals accessible
fix(modal): Make x to close react modals accessible

By hiding it for screen readers
[Finishes #100277796]

sjolicoeur/pivotal-ui,pivotal-cf/pivotal-ui,sjolicoeur/pivotal-ui,sjolicoeur/pivotal-ui,sjolicoeur/pivotal-ui,pivotal-cf/pivotal-ui,pivotal-cf/pivotal-ui,CanopyCloud/pivotal-ui,npm/pivotal-ui,CanopyCloud/pivotal-ui,npm/pivotal-ui,npm/pivotal-ui,CanopyCloud/pivotal-ui