move not_selected from external tools settings to it's own column

fixes: PLAT-646

Test Plan:
 * Install an external tool with the property "not_selectable" as true
 * when you look in the asmnt/module item external tool list the tool
   should not be listed

Change-Id: If66e09b83373fad83274d0120cb464e7caf60c83
Reviewed-on: https://gerrit.instructure.com/41205
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Clare Strong <clare@instructure.com>
Reviewed-by: Bracken Mosbacker <bracken@instructure.com>
Product-Review: Bracken Mosbacker <bracken@instructure.com>