Make processing redirects easier to override

This MR moves the logic for tracking events and validating urls into the experiment class, where they probably should've always been.

So, part of what I think has been a struggle with this middleware is that we tried to do it as a "one off" kinda deal.. just a quick drop in, and ship it.

But it's turned out that was naive, and we should've added the logic to the experiment class. The reason is that it can be overridden and handled / overridden very easily when we do this. I guess this didn't surface entirely for me until @jayswain asked a couple questions this morning, and so, we end up with a more flexible implementation, and one we probably should've hit initially.

Anyway, we got there! That's the important part. Right? 😅