Woocommerce GoCardless 2.6.4

* Fix - Error when adding payment method via /my-account/add-payment-method/ page.
* Enhancement - Added support to replace mandate when there's `mandate_replaced` event from webhook.

* Tweak - Default subscription order and renewal order to processing. Filters provided to override the initial order status.
* Fix - Make sure to check if `get_current_screen` function exists in which the wrapper might be called in non-admin page.
* Fix - Incorrect filter name for create redirect flow params.

* Fix - Added support for WooCommerce 3.0 and compatibility with older versions.
* Fix - Fatal error caused by calling empty cart during renewal order.