link to discussion topic and quiz menu tools on assignment index

test plan:
* enable the lor feature flag
* create an external tool with quiz_menu and
discussion_topic_menu configured
* should show external tool links specific to each type
on the assignments index page for graded discussion topics
and quiz assignments

closes #CNVS-15520

Change-Id: I020ab9cbc583d7940b0051056d27acb557ab152c
Reviewed-on: https://gerrit.instructure.com/41172
Reviewed-by: Jeremy Stanley <jeremy@instructure.com>
QA-Review: Clare Strong <clare@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
Product-Review: Bracken Mosbacker <bracken@instructure.com>