Add image descriptions to title attribute to view on mouse hover/long-press. (#5137)

* Add image descriptions to `title` attribute to view on mouse hover/long-press.

* Too many title properties may spoil the broth.

app/javascript/mastodon/components/media_gallery.js

@@ -135,7 +135,7 @@ class Item extends React.PureComponent {
           onClick={this.handleClick}
           target='_blank'
         >
-          <img src={previewUrl} srcSet={srcSet} sizes={sizes} alt={attachment.get('description')} />
+          <img src={previewUrl} srcSet={srcSet} sizes={sizes} alt={attachment.get('description')} title={attachment.get('description')} />
         </a>
       );
     } else if (attachment.get('type') === 'gifv') {