MM-11703: Update incoming and outgoing webhook description to 500 characters (#9265)

* MM-11703: Update incoming and outgoing webhook description to 500 characters

* MM-11703: Add upgrade code

* MM-11703: Remove MAX_DESCRIPTION_LENGTH constants

* MM-11703: GOFMT

* MM-11703: Move upgrade code to version 5.3

* MM-11703: Comment back upgrade to 5.3.0

* MM-11703: Add API errors back

* MM-11703: Copy-Paste

model/incoming_webhook.go

@@ -93,7 +93,7 @@ func (o *IncomingWebhook) IsValid() *AppError {
 		return NewAppError("IncomingWebhook.IsValid", "model.incoming_hook.display_name.app_error", nil, "", http.StatusBadRequest)
 	}
 
-	if len(o.Description) > 128 {
+	if len(o.Description) > 500 {
 		return NewAppError("IncomingWebhook.IsValid", "model.incoming_hook.description.app_error", nil, "", http.StatusBadRequest)
 	}