add content parameters to course_home_sub_navigation lti launch

fixes CNVS-13798

test plan:
 - copy the xml configuration for the lti-tool-provider example
   lti app to a local file.
 - modify the xml by duplicating the migration_selection
   extension, changing the extension name to
   course_home_sub_navigation
 - configure the lti app on a course by pasting the modified xml
 - navigate to the course home page and open the network tab on
   the web inspector
 - launch the tool by clicking on its button on the right
   sidebar
 - inspect the POST request to the lti app and verify that 4
   "ext_content_" parameters are present:
   - intended_use
   - return_types
   - file_extensions
   - return_url
 - click on one of the supported links
 - should be redirected to the "tool use finished" page.

Change-Id: If5df5657c64377fdb2badca1d890c73e95548fbc
Reviewed-on: https://gerrit.instructure.com/37834
Reviewed-by: Brad Humphrey <brad@instructure.com>
QA-Review: Clare Strong <clare@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
Product-Review: Bracken Mosbacker <bracken@instructure.com>