Make the message when the user returns from the gateway (onReturn method) optional

Motivation

In my setup I redirect the users to a page where the success of their payment is explained so the message "'Your payment has been completed successfully.'" is redundant.

Steps to reproduce

Complete a transaction in the gateway and back in the Drupal site a message is always displayed.

Proposed resolution

Add a new configuration for the payment gateway to personalize that message. Do not display it when is empty.

Remaining tasks

Consider this new feature then code it.

User interface changes

New option in the gateway form.

API changes

Data model changes