Adds react-modal to Canvas

Test Plan:
  - Nothing really testable, since it just adds a library

Change-Id: Ib5977abeffdc759a57c5373e193a9b113af54a49
Reviewed-on: https://gerrit.instructure.com/41835
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Jason Madsen <jmadsen@instructure.com>
Product-Review: Jason Madsen <jmadsen@instructure.com>
QA-Review: Jason Madsen <jmadsen@instructure.com>