warn when crocodoc session is about to expire

closes CNVS-12571

Test plan:
  * submit a crocodocable homework assignment
  * open the assignment in the speedgrader
  * you should be periodically warned that your session is expiring
    after 50 minutes

Change-Id: Ic4a72e19b5e8fbd9e65df368c6f78ae486fcd197
Reviewed-on: https://gerrit.instructure.com/40278
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Simon Williams <simon@instructure.com>
QA-Review: Anna Koalenz <akoalenz@instructure.com>
Product-Review: Cameron Matheson <cameron@instructure.com>