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/features/notifications/components/notification.jsx

@@ -71,7 +71,7 @@ const Notification = React.createClass({
     );
   },
 
-  render () {
+  render () { // eslint-disable-line consistent-return
     const { notification } = this.props;
     const account          = notification.get('account');
     const displayName      = account.get('display_name').length > 0 ? account.get('display_name') : account.get('username');