Increase scope of TRACK_IGNORE_URLS

This implements the second option described in https://github.com/bruth/django-tracking2/issues/45. Simply put this makes tracking2 (essentially) oblivious to any URL that is defined in `TRACK_IGNORE_URLS`. Previously it would track the Visitor but not create a Pageview, now it does not track the Visitor (nor create a Pageview).

I also refactored the middleware itself a bit to split up the `should_track`, `refresh_visitor`, and `add_pageview` parts.