Previously, using the bulk delete tool in the Django admin for
`CertificateDateOverride`s would not trigger the post to the credentials
service to delete the corresponding date override stored there. This was
because we were sending the COURSE_CERT_CHANGED signal from an
overridden version of the delete() method. But the Django docs warn:
"Note that the delete() method for an object is not necessarily called
when deleting objects in bulk using a QuerySet or as a result of a
cascading delete. To ensure customized delete logic gets executed, you
can use pre_delete and/or post_delete signals."
Now we use the post_delete signal instead of overriding the delete()
method to make sure the COURSE_CERT_CHANGED signal is sent for both
individual and bulk deletes.
MICROBA-1487
703c917f2a