docs: add CertificateModel ADR. (#31825)

Adds an ADR explaining that historical fields
used by CertificateModel were not removed.

Adds comments to point to ADR.

lms/djangoapps/certificates/docs/decisions/008-certificate-model-remnants.rst

@@ -0,0 +1,44 @@
+Leaving PDF Certificate Fields in Certificates Model
+====================================================
+
+Status
+------
+
+Accepted
+
+Context
+-------
+
+As part of the `deprecation of PDF certificates`_, we had to make a decision about the future of several existing fields in
+the ``GeneratedCertificate`` model that would no longer be used by the UI code. The replacement for PDF certificates is
+web/HTML certificates, which have been outlined in a `previous ADR`_.
+
+
+.. _deprecation of PDF certificates: https://github.com/openedx/public-engineering/issues/27
+.. _previous ADR: https://github.com/openedx/edx-platform/blob/master/lms/djangoapps/certificates/docs/decisions/003-web-certs.rst
+
+Decision
+--------
+
+We decided to leave the existing fields ``download_url``, ``download_uuid``, and ``error_reason`` within the ``GeneratedCertificate``
+model and did not remove them as part of our deprecation process where we removed the UI to display them.
+
+
+Consequences
+------------
+
+A consequence of this decision is that all Open edX operators, including edx.org, will continue to store this data in their databases,
+even though it does not have any functional use. 
+
+Several pieces of code still reference these fields, and there are events that still pass around the download_url, even though this field
+should no longer be populated or used any longer.
+
+
+Alternatives Considered
+-----------------------
+
+We considered removing these fields and all associated code, but it was felt as though it was going to be too complex, because it involved
+migrations and several API changes in order to support the removal.
+
+We also felt as though it would be useful for operators that did have historical PDF certificates to continue to have a historical record of
+where the PDF certificates were stored.

lms/djangoapps/certificates/models.py

@@ -224,8 +224,6 @@ class GeneratedCertificate(models.Model):
     user = models.ForeignKey(User, on_delete=models.CASCADE)
     course_id = CourseKeyField(max_length=255, blank=True, default=None)
     verify_uuid = models.CharField(max_length=32, blank=True, default='', db_index=True)
-    download_uuid = models.CharField(max_length=32, blank=True, default='')
-    download_url = models.CharField(max_length=128, blank=True, default='')
     grade = models.CharField(max_length=5, blank=True, default='')
     key = models.CharField(max_length=32, blank=True, default='')
     distinction = models.BooleanField(default=False)
@@ -234,6 +232,11 @@ class GeneratedCertificate(models.Model):
     name = models.CharField(blank=True, max_length=255)
     created_date = models.DateTimeField(auto_now_add=True)
     modified_date = models.DateTimeField(auto_now=True)
+
+    # These fields have been kept around even though they are not used.
+    # See lms/djangoapps/certificates/docs/decisions/008-certificate-model-remnants.rst for the ADR.
+    download_uuid = models.CharField(max_length=32, blank=True, default='')
+    download_url = models.CharField(max_length=128, blank=True, default='')
     error_reason = models.CharField(max_length=512, blank=True, default='')
 
     # This is necessary because CMS does not install the certificates app, but it