Pa11ycrawler

Now with pa11y ignore rules! See 43

- Compatibility with older versions of Node.js
- Switch back to committing assets to the repo, instead of loading from NPM

https://github.com/edx/pa11ycrawler/pull/37
- Use pa11y's up-to-date JSON reporter
- Display CSS selector for pa11y violations
- Switch templating system from Mako to Jinja2
- Load assets via NPM instead of committing them to repo

Now pa11ycrawler is smart enough to automatically log back in if it gets logged out!

Packaging fix. See 15.