ensure template history serializes using template serialize fn

app/models.py

@@ -506,18 +506,7 @@ class TemplateHistory(db.Model):
                              default=NORMAL)
 
     def serialize(self):
-        serialized = {
-            "id": self.id,
-            "type": self.template_type,
-            "created_at": self.created_at.strftime(DATETIME_FORMAT),
-            "updated_at": self.updated_at.strftime(DATETIME_FORMAT) if self.updated_at else None,
-            "created_by": self.created_by.email_address,
-            "version": self.version,
-            "body": self.content,
-            "subject": self.subject if self.template_type == EMAIL_TYPE else None
-        }
-
-        return serialized
+        return Template.serialize(self)
 
 
 MMG_PROVIDER = "mmg"