fix: add created and modified fields to integrity signature admin (#27882)

MST-861. The Django admin page for IntegritySignature only displays
the user and course_key for a signature. This adds the created and modified
fields, and will order signatures by the modified field.

openedx/core/djangoapps/agreements/admin.py

@@ -11,9 +11,10 @@ class IntegritySignatureAdmin(admin.ModelAdmin):
     """
     Admin for the IntegritySignature Model
     """
-    list_display = ('user', 'course_key',)
-    readonly_fields = ('user', 'course_key',)
+    list_display = ('user', 'course_key', 'created', 'modified')
+    readonly_fields = ('user', 'course_key', 'created', 'modified')
     search_fields = ('user__username', 'course_key',)
+    ordering = ['-modified']
 
     class Meta:
         model = IntegritySignature