tornevalls-resurs-bank-payment-gateway-for-woocommerce

RWC-436,
Close #107.

We should honor WOO-690 with this issue.

* Update: in is_available() checks are breaking earlier if $this->paymentMethodInformation->id is unexistent due to dependency.
* Update: getOrderButtonHtml() can return null for unknown reasons. Now, we always cast it as string before return.
* Update: getResursOrderIfExists() should always return array, instead of null (expected).
* Add: Action woocommerce_order_status_completed for early completion. May not have effect at all (but we save the commit).
* Add: Method for completed status - updateOrderStatusByCompletion - which throws an early error if the order is frozen at Resurs.
* Update: handleOrderByNewSlug now handles $oldSlug too, in case the order status has to be "reset".
* Update: If order is frozen in the slugcheck, we not only throw an exception (as protest), we also restore the last slug (with $oldSlug).