Access a public course while logged out

test plan:
 - launch a navigation tool
 - launch a module item tool
 - launch a tool using /courses/:course_id/external_tools/retrieve?url=:launch_url
  * they should all launch

 - retest with a private course
  * they should all redirect to the login page

 - verify that PLAT-572 is fixed

fixes PLAT-678 PLAT-572

Change-Id: I9ccfd990b14428053116f96eb064897dd3dd9262
Reviewed-on: https://gerrit.instructure.com/42625
Reviewed-by: Bracken Mosbacker <bracken@instructure.com>
Product-Review: Bracken Mosbacker <bracken@instructure.com>
QA-Review: Bracken Mosbacker <bracken@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Nathan Mills <nathanm@instructure.com>