feat: add error message page; improve admin ui; add missing test files

aircox/models/page.py

@@ -278,6 +278,16 @@ class Comment(models.Model):
     date = models.DateTimeField(auto_now_add=True)
     content = models.TextField(_("content"), max_length=1024)
 
+    item_template_name = "aircox/widgets/comment_item.html"
+
+    @cached_property
+    def parent(self):
+        """Return Page as its subclass."""
+        return Page.objects.select_subclasses().filter(id=self.page_id).first()
+
+    def get_absolute_url(self):
+        return self.parent.get_absolute_url()
+
     class Meta:
         verbose_name = _("Comment")
         verbose_name_plural = _("Comments")