extract user note  alert check to it's own class

    fixes CNVS-12402

    test-plan:
    regression check user note alerts for timespan threshold

Change-Id: I33abc799d7c34185aba15e62387eedee9b1acaf6
Reviewed-on: https://gerrit.instructure.com/33295
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>