Add revoke program certificate task.

Upon invalidating course certificate, revoke related program certificates
as well.

PROD-1271

lms/djangoapps/certificates/models.py

@@ -74,7 +74,7 @@ from badges.events.course_meta import completion_check, course_group_check
 from course_modes.models import CourseMode
 from lms.djangoapps.instructor_task.models import InstructorTask
 from openedx.core.djangoapps.content.course_overviews.models import CourseOverview
-from openedx.core.djangoapps.signals.signals import COURSE_CERT_AWARDED, COURSE_CERT_CHANGED
+from openedx.core.djangoapps.signals.signals import COURSE_CERT_AWARDED, COURSE_CERT_CHANGED, COURSE_CERT_REVOKED
 from openedx.core.djangoapps.xmodule_django.models import NoneToEmptyManager
 from util.milestones_helpers import fulfill_course_milestone, is_prerequisite_courses_enabled
 
@@ -369,8 +369,14 @@ class GeneratedCertificate(models.Model):
         self.download_url = ''
         self.grade = ''
         self.status = CertificateStatuses.unavailable
-
         self.save()
+        COURSE_CERT_REVOKED.send_robust(
+                sender=self.__class__,
+                user=self.user,
+                course_key=self.course_id,
+                mode=self.mode,
+                status=self.status,
+            )
 
     def mark_notpassing(self, grade):
         """