add tool links to gear menus on discussion topics

test plan:
* as with g/39874, add an external tool to a course
with 'discussion_topic_menu' configured

* enable lor feature flag for account or user
* confirm that menu items for the tool are added to the
 gear menus on the index and individual show pages for
 discussion topics

closes #CNVS-15239

Change-Id: Ic309c14f75ae0b2e38f9d1324b7a8d370fa93fde
Reviewed-on: https://gerrit.instructure.com/40550
QA-Review: Clare Strong <clare@instructure.com>
Reviewed-by: Jeremy Stanley <jeremy@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
Product-Review: Bracken Mosbacker <bracken@instructure.com>