add aria label and button role to external tool buttons

fixes PLAT-587

test-plan:
the edit and delete tool buttons on the External tabs page should be readable by NVDA and screenreaders

Change-Id: I090c43137d49189ba20469ef54aad6b6833b7d2e
Reviewed-on: https://gerrit.instructure.com/38860
Reviewed-by: Brad Humphrey <brad@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Clare Strong <clare@instructure.com>
Product-Review: Nathan Mills <nathanm@instructure.com>