Add eslint-plugin-jsx-a11y (#1651)

* Add eslint-plugin-jsx-a11y.

* Fix npm script.

* Adjust npm scripts so test also runs lint.

* Fix existing lint errors.

* Don't break on a11y issues.

* Add role and tabIndex.

* Add vim and Mac files to .gitignore and .dockerignore.

* Handle htmlFor (partially), a that's actually a button.

* Fix missing tabIndex.

* Add cursor:pointer to load-more

* Revert change to load_more.

* Fixes based on review.

* Update yarn.lock.

* Don't try to install fsevents on Linux (hides warning noise).

app/assets/javascripts/components/components/button.jsx

@@ -9,6 +9,7 @@ const Button = React.createClass({
     block: React.PropTypes.bool,
     secondary: React.PropTypes.bool,
     size: React.PropTypes.number,
+    style: React.PropTypes.object,
     children: React.PropTypes.node
   },