Update react-modal and establish canvas modal style

fixes PLAT-771

Change-Id: Ia7ea6ca7002f1c1926f845d74af3544337c96def
Reviewed-on: https://gerrit.instructure.com/44584
Tested-by: Jenkins <jenkins@instructure.com>
Product-Review: Colleen Palmer <colleen@instructure.com>
Reviewed-by: Colleen Palmer <colleen@instructure.com>
QA-Review: Eric Berry <ericb@instructure.com>