Allow query parameters in handler.flow_entry_link

From the original pull request:
> I got to the point while refactoring ItemAlreadyBought (remember that? ;) ), where I needed to pass the item's URL in some cases into a different flow. Neither url_args nor url_kwargs seemed to be good fits for that, so I opted for the simplest possible solution by allowing the caller to add query parameters to the flow URL.

This time I omitted all the LP-only code changes for your convenience :).