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/permalink.jsx

@@ -6,7 +6,8 @@ const Permalink = React.createClass({
 
   propTypes: {
     href: React.PropTypes.string.isRequired,
-    to: React.PropTypes.string.isRequired
+    to: React.PropTypes.string.isRequired,
+    children: React.PropTypes.node.isRequired
   },
 
   handleClick (e) {