make Notification.find work with string ids

fixes CNVS-15695

test plan
- running in production mode, cause a notification to be sent
- ensure the notification sends

Change-Id: Ie98cca4bc78597327d4199b1b6f64f2f0bfe12be
Reviewed-on: https://gerrit.instructure.com/41448
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Alex Boyd <aboyd@instructure.com>
Product-Review: Matthew Wheeler <mwheeler@instructure.com>
QA-Review: Steven Shepherd <sshepherd@instructure.com>