queue content migration through content-item launch

test plan:
* set up a course_home_sub_navigation enabled tool, as in
 the test plan for g/37834
* launch the tool from the course home page
* select the imscc file on the test tool content
 selection page
* should be redirected to the content_migration page
 and have a new migration queued for the course

closes #CNVS-13797

Change-Id: If94e65d2ba3c1f2ed53624b6a3b4c9a75d25c5ce
Reviewed-on: https://gerrit.instructure.com/38206
Reviewed-by: Jon Willesen <jonw@instructure.com>
Product-Review: Jon Willesen <jonw@instructure.com>
QA-Review: Clare Strong <clare@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>