add feature flags for LOR external tools

test plan:
* add an external tool configured for the extensions
 going to be used by LOR (:course_settings_sub_navigation,
 :course_home_sub_navigation, :migration_selection)
* confirm that it is not shown to a user unless either
 the account-level feature flag is enabled or the
 user-level flag is enabled

closes #CNVS-14626

Change-Id: I06e189fc1f2d745613353ea1b3f98b4f4b2c87b6
Reviewed-on: https://gerrit.instructure.com/39111
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Jeremy Stanley <jeremy@instructure.com>
QA-Review: Clare Strong <clare@instructure.com>
Product-Review: Bracken Mosbacker <bracken@instructure.com>