Disable audit certificates for new audit enrollments.

Two new certificate statuses are introduced, 'audit_passing' and
'audit_notpassing'. These signal that the GeneratedCertificate is not
to be displayed as a cert to the user, and that they either passed or
did not. This allows us to 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.

ECOM-3040
ECOM-3515

common/djangoapps/course_modes/models.py

@@ -592,6 +592,18 @@ class CourseMode(models.Model):
         modes = cls.modes_for_course(course_id)
         return min(mode.min_price for mode in modes if mode.currency.lower() == currency.lower())
 
+    @classmethod
+    def is_eligible_for_certificate(cls, mode_slug):
+        """
+        Returns whether or not the given mode_slug is eligible for a
+        certificate. Currently all modes other than 'audit' grant a
+        certificate. Note that audit enrollments which existed prior
+        to December 2015 *were* given certificates, so there will be
+        GeneratedCertificate records with mode='audit' which are
+        eligible.
+        """
+        return mode_slug != cls.AUDIT
+
     def to_tuple(self):
         """
         Takes a mode model and turns it into a model named tuple.

common/djangoapps/course_modes/tests/test_models.py

@@ -430,3 +430,16 @@ class CourseModeModelTest(TestCase):
         verified_mode.expiration_datetime = None
         self.assertFalse(verified_mode.expiration_datetime_is_explicit)
         self.assertIsNone(verified_mode.expiration_datetime)
+
+    @ddt.data(
+        (CourseMode.AUDIT, False),
+        (CourseMode.HONOR, True),
+        (CourseMode.VERIFIED, True),
+        (CourseMode.CREDIT_MODE, True),
+        (CourseMode.PROFESSIONAL, True),
+        (CourseMode.NO_ID_PROFESSIONAL_MODE, True),
+    )
+    @ddt.unpack
+    def test_eligible_for_cert(self, mode_slug, expected_eligibility):
+        """Verify that non-audit modes are eligible for a cert."""
+        self.assertEqual(CourseMode.is_eligible_for_certificate(mode_slug), expected_eligibility)