move ungraded count checking to it's own class

fixes CNVS-12392

test plan:
regression test on alerts for ungraded assignments

Change-Id: I0ee553666706e91a96b953b1796578b834fd88b7
Reviewed-on: https://gerrit.instructure.com/33284
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>