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/media_gallery.jsx

@@ -220,7 +220,7 @@ const MediaGallery = React.createClass({
       }
 
       children = (
-        <div style={spoilerStyle} className='media-spoiler' onClick={this.handleOpen}>
+        <div role='button' tabIndex='0' style={spoilerStyle} className='media-spoiler' onClick={this.handleOpen}>
           <span style={spoilerSpanStyle}>{warning}</span>
           <span style={spoilerSubSpanStyle}><FormattedMessage id='status.sensitive_toggle' defaultMessage='Click to view' /></span>
         </div>