Enables an HTML template for the standard user registration welcome email.

This is the most basic and natural implementation of an HTML template for the user registration 'welcome' email. That which is dispatched when a customer registers a new user account, either on the registration page or during the checkout process. Frankly, I'm unsure why it wasn't implemented in this way in the core. There are no special features, no admin control panel or editor. Theme developers can use this to simply fashion their own HTML email template just as they would the Order Confirmation email, which is templated by default.