Parse ReturnURL to append order id correctly

Make it work whether or not the configured URL already has a querystring.
Once this is deployed, add ?ffname=amazon to $wgAmazonGatewayReturnURL to
make the logspam end

Change-Id: I890dff2ab9e19b464ae5ab8173c3b62a16adc84a
Merge "Parse ReturnURL to append order id correctly"