Disable audit certificates for new audit enrollments.

An `eligible_for_certificate` field is added to the
GeneratedCertificate model. This way we can retain existing grading
logic, as well as maintaining correctness in analytics and reporting.

Ineligible certificates are hidden by using the
`eligible_certificates` manager on GeneratedCertificate. Some places
in the coe (largely reporting, analytics, and management commands) use
the default `objects` manager, since they need access to all
certificates.

This commit also updates the DB cache for acceptance tests.

lms/djangoapps/instructor_task/tasks_helper.py

@@ -1584,7 +1584,7 @@ def invalidate_generated_certificates(course_id, enrolled_students, certificate_
     :param enrolled_students: (queryset or list) students enrolled in the course
     :param certificate_statuses: certificates statuses for whom to remove generated certificate
     """
-    certificates = GeneratedCertificate.objects.filter(
+    certificates = GeneratedCertificate.objects.filter(  # pylint: disable=no-member
         user__in=enrolled_students,
         course_id=course_id,
         status__in=certificate_statuses,

lms/djangoapps/instructor_task/tests/test_tasks_helper.py

@@ -1802,7 +1802,7 @@ class TestCertificateGeneration(InstructorTaskModuleTestCase):
             },
             result
         )
-        generated_certificates = GeneratedCertificate.objects.filter(
+        generated_certificates = GeneratedCertificate.eligible_certificates.filter(
             user__in=students,
             course_id=self.course.id,
             mode='honor'
@@ -1912,7 +1912,7 @@ class TestCertificateGeneration(InstructorTaskModuleTestCase):
             result
         )
 
-        generated_certificates = GeneratedCertificate.objects.filter(
+        generated_certificates = GeneratedCertificate.eligible_certificates.filter(
             user__in=students,
             course_id=self.course.id,
             mode='honor'