- 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.
Based on feedback from first-time contributors, this commit simplifies
the number of paths available for finding the dev setup directions you
need. Setup instructions are now organized into Recommended (Vagrant)
and Advanced (non-Vagrant).
In order to get the ToC to display correctly, I've combined all the
advanced, non-Vagrant methods into one page. I've left the old pages
intact, with a note that the content has been moved and link to the new
page. This is in case folks have linked directly to those pages.
The advanced setup directions still need to be cleaned up, but this is a
start.
