Improve, fix, WC_Payment_Gateway::get_transaction_url()

* Make the new get_transaction_url() method a bit more
flexible/convenient by passing in the full order object rather than
transaction_id
* Fix bug with core PayPal gateway get_transaction_url() override
Merge pull request #6113 from justinstern/master

Improve, fix, WC_Payment_Gateway::get_transaction_url()