Add course settings sub navigation lti extension

fixes RD-76

test plan:
 - as a teacher, navigate to a course settings page. the right
   sidebar should look normal.
 - modify an lti app's xml to include the
   course_settings_sub_navigation extension
 - configure an lti app via modified xml.
 - navigate to a course settings page, where you should see a
   button for the configured tool at the top of the right
   sidebar.
 - clicking on the button should launch the tool.

Change-Id: Iec9e3337437295865bab66a499fe8fc096ad0ba5
Reviewed-on: https://gerrit.instructure.com/36471
Reviewed-by: Brad Humphrey <brad@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Nathan Rogowski <nathan@instructure.com>
Product-Review: Jon Willesen <jonw@instructure.com>