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/status_content.jsx

@@ -119,7 +119,7 @@ const StatusContent = React.createClass({
       return (
         <div className='status__content' style={{ cursor: 'pointer' }} onMouseDown={this.handleMouseDown} onMouseUp={this.handleMouseUp}>
           <p style={{ marginBottom: hidden && status.get('mentions').size === 0 ? '0px' : '' }} >
-            <span dangerouslySetInnerHTML={spoilerContent} />  <a className='status__content__spoiler-link' onClick={this.handleSpoilerClick}>{toggleText}</a>
+            <span dangerouslySetInnerHTML={spoilerContent} />  <a tabIndex='0' className='status__content__spoiler-link' role='button' onClick={this.handleSpoilerClick}>{toggleText}</a>
           </p>
 
           {mentionsPlaceholder}