content migration import-in-progress notice

test plan:
 - in an environment with redis caching enabled,
  - create a content migration with the API and add the
    undocumented parameter
     settings[import_in_progress_notice]=1
  - while the course import is in progress, course pages
    should show a blue banner at the top of each page
    indicating that an import is in progress, and linking
    to the migration status page

fixes CNVS-14517

Change-Id: I9cf5c10e043f07460a1e42491addd029cfe31396
Reviewed-on: https://gerrit.instructure.com/38815
Reviewed-by: James Williams  <jamesw@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Clare Strong <clare@instructure.com>
Product-Review: Cosme Salazar <cosme@instructure.com>