Fix assignment missing periodic job
Fix assignment missing periodic job

Refs MBL-10482

Test Plan:
 - Create an observer alert threshold for
   assignment_missing between an observer and student
 - Create an assignment that is due in 1-2 minutes
 - Wait at least 5 minutes
 - Hit /api/v1/users/self/observer_alerts
 - An assignment_missing alert should exist

Change-Id: I7936d1eb7bcf347a776c3d0af675e61f5c25db98
Reviewed-on: https://gerrit.instructure.com/153446
Reviewed-by: Nate Armstrong <619b324efbf5f7848de055bdc8bef4c0dd7c8ef5@instructure.com>
QA-Review: Nate Armstrong <619b324efbf5f7848de055bdc8bef4c0dd7c8ef5@instructure.com>
Tested-by: Jenkins
Product-Review: Matthew Sessions <c7196251943e6fd2cf96a68670e4730c7e7c63b5@instructure.com>

sfu/canvas-lms,instructure/canvas-lms,sfu/canvas-lms,instructure/canvas-lms,instructure/canvas-lms,djbender/canvas-lms,sfu/canvas-lms,djbender/canvas-lms,sfu/canvas-lms,djbender/canvas-lms,instructure/canvas-lms,sfu/canvas-lms,instructure/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,djbender/canvas-lms,instructure/canvas-lms,instructure/canvas-lms