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/clear_column_button.jsx

@@ -15,7 +15,7 @@ const ClearColumnButton = React.createClass({
     const { intl } = this.props;
 
     return (
-      <div title={intl.formatMessage(messages.clear)} className='column-icon column-icon-clear' tabIndex='0' onClick={this.props.onClick}>
+      <div role='button' title={intl.formatMessage(messages.clear)} className='column-icon column-icon-clear' tabIndex='0' onClick={this.props.onClick}>
         <i className='fa fa-eraser' />
       </div>
     );