redirect after login to "next" login param

After login, in thanks page, redirect user to "next" get param that was sent to login
`.get` on a dict returns `None` if the key isn't found

```python
request.session["next_url"] = request.GET.get('next')
```

`.get` takes a second param that allows you to return a value if the key (`next_url`) is Falsee
```
redirect_url = request.session.get('next_url', redirect_url)  # if next_url doesn't exist in request.session then we just return whatever redirect_url currently is
return HttpResponseRedirect(redirect_url)
```