Clean up various admin templates.<br />Adds support for a packaging module type.<br />Shipping modules are now detected by modules implementing the \CGExtensions\shipping_module interface. Some shipping modules may be broken<br />Payment gateway modules are now detected by modules implementing the \CGExtensions\payment_gateway interface. Some payment gateway modules may be broken.<br />Adds various classes, including a base shipping estimate class, packing list classes, new interfaces...<br />Code cleanup.