Allow query parameters in handler.flow_entry_link

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 .

Sorry about the 'whitespace fix' commit, I can't get IDEA to ignore whitespace settings for individual modules in a project, so that ended up in the pull request.