add layout for borderless LTI launches

Test Plan
 - Visit /courses/:course_id/external_tools/retrieve?url=:launch_url&borderless=1
 - There should be no insecure warning for an https launch

fixes PLAT-581

Change-Id: I050b93f5f4f1fc82f8039a29cde802addc734fcd
Reviewed-on: https://gerrit.instructure.com/38757
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Nathan Mills <nathanm@instructure.com>
QA-Review: Clare Strong <clare@instructure.com>
Product-Review: Bracken Mosbacker <bracken@instructure.com>