EDUCATOR-171

https://openedx.atlassian.net/browse/TNL-5008

The previous logic was a convoluted way of doing the same thing, and
has already led to one bug. Instead of hoping that the bugs are ironed
out now and that future devs maintain this logic properly, let's just
set a real cutoff date.

COR-2403

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

This reverts commit 60860e3a.

This reverts commit 54c349b4.

This reverts commit ee13ffd1.

ECOM-3401

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.

Please note that this is a squshed commit and the work of:
Symbolist, macdiesel, nedbat, doctoryes, muzaffaryousaf and muhammad-ammar

Fixing certificates for credit mode.
ECOM-1988

improved coverage

refactoring based on feedback

fixed quality violations

changes based on feedback

Add the ability to generate "example" certificates to
test that certificate generation is working correctly for a course.

Add the ability to enable/disable self-generated certificates
on a per-course basis.