stringify outcome result rollup ids

fixes CNVS-10268

test plan
- fetch outcome result rollups from the api
- ensure that all id fields are stringified

Change-Id: I17950ae4fc82278d34345c96b6b6c49815668977
Reviewed-on: https://gerrit.instructure.com/28090
Reviewed-by: Jon Willesen <jonw@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Matt Fairbourn <mfairbourn@instructure.com>
Product-Review: Joel Hough <joel@instructure.com>