refactor Alert::UserNote to check enable_user_notes on account

fixes CNVS-12406

test plan
regression test alerts sent based on user notes and the enable user
notes accout setting

Change-Id: I9627c7db5381be282f6ed9134aee2f44fef76720
Reviewed-on: https://gerrit.instructure.com/33306
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>