extract ungraded timespan alert check to it's own class

fixes CNVS-12393

test-plan:
regression check ungraded submission alerts for timespan threshold

Change-Id: I228c2e152d20a61871a483974e5a418a9b61e56d
Reviewed-on: https://gerrit.instructure.com/33287
Reviewed-by: Cody Cutrer <cody@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Jeremy Putnam <jeremyp@instructure.com>
Product-Review: Simon Williams <simon@instructure.com>