Implement querystring passthrough parameter [RD-20]

Somewhat intelligently handles query strings added by third parties.

The example I was discussing with @almathew took up the bulk of this time. The order of operations had to be reversed in that instance to preserve correct query string separators (we should return tracker.com?http://job.com?foo=bar, for instance, which is correct even though it looks wrong; without reversing the order, we would return tracker.com?http://job.com&foo=bar, which seems to break doubleclick).

All tests pass, as do several manual tests using jobs I thought might break.