✨(admin) add certificate definition field into order admin change view

On certificate admin change view, we add a custom field to display the
certificate definition which the certificate relies on.
In order admin change, we also display the certificate if there is.

src/backend/joanie/core/admin.py

@@ -55,6 +55,19 @@ class CertificateAdmin(admin.ModelAdmin):
     """Admin class for the Certificate model"""
 
     list_display = ("order", "issued_on")
+    readonly_fields = ("order", "issued_on", "certificate_definition")
+
+    def certificate_definition(self, obj):  # pylint: disable=no-self-use
+        """Retrieve the certification definition from the related order."""
+        certificate_definition = obj.order.product.certificate_definition
+
+        return format_html(
+            (
+                f"<a href='{reverse('admin:core_certificatedefinition_change', args=(certificate_definition.id,), )}'>"
+                f"{str(certificate_definition)}"
+                "</a>"
+            )
+        )
 
 
 @admin.register(models.Course)
@@ -260,7 +273,7 @@ class OrderAdmin(DjangoObjectActions, admin.ModelAdmin):
     """Admin class for the Order model"""
 
     list_display = ("uid", "owner", "product", "state")
-    readonly_fields = ("total", "invoice")
+    readonly_fields = ("total", "invoice", "certificate")
     change_actions = (ACTION_NAME_GENERATE_CERTIFICATES,)
     actions = (ACTION_NAME_CANCEL, ACTION_NAME_GENERATE_CERTIFICATES)