refactor lti setting/selection_type to placements

 - specs should pass

Change-Id: Iefdeb4b4dbc4a878fd11c7b4dec8caa81c741084
Reviewed-on: https://gerrit.instructure.com/42922
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Nathan Mills <nathanm@instructure.com>
Product-Review: Bracken Mosbacker <bracken@instructure.com>
QA-Review: Brad Humphrey <brad@instructure.com>