make all_tools_for return a scope

fixes: PLAT-650

test-plan:
*you should still be able to select a external tool for a module item
*the list external tools api should still work

Change-Id: I6ec4826b4efc92f33ceb5852ed9dbb0d971f3590
Reviewed-on: https://gerrit.instructure.com/41403
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Brad Humphrey <brad@instructure.com>
QA-Review: Clare Strong <clare@instructure.com>
Product-Review: Nathan Mills <nathanm@instructure.com>