Fix #248: Fix next param logic and move logic to Python.

By deciding whether to redirect the user to the next param URL or the
default URL in Python instead of JavaScript, we can rely on Django’s
is_safe_url logic.
That's it! Burn everything and start over and rewrite it in VBA. I can't work on this. And besides, we should not use vowels in any variable names too is my new must-have rule.