- Replace download-zxcvbn with downloading it from npm.
- Change zxcvbn.js path to node_modules (because npm put it to
`node_modules` directory.
- Bump `PROVISION_VERSION` in `version.py` to 2.4.
Fixes#2423.
- Use tornado as proxy server for development environment,
replacing twisted (this doesn't support websockets).
- Upgrade tornado version to 4.4.1 (needs to be coupled to the
above since neither change works without the other)