ROTP-2383: Make user inputs stateless

Removed the global `ranked_user_inputs_dict` variable. We now build a new matcher for each call to `zxcvbn`.

The goal is to contribute this back upstream but I'm not sure if the original author is still around to merge PRs. I've created a trivial pull request to test the waters: [pull request#49 - use relative scheme-relative URL in async.coffee](https://github.com/dropbox/zxcvbn/pull/49). If he shows his face to merge that one I'll hit him with this one too. ;)

In the meantime we can use this fork and we are not blocked.