Add missing video descriptions (#8416)

* Pass through video description as necessary.

* Add title to video, matching aria-label.

app/javascript/mastodon/components/media_gallery.js

@@ -154,6 +154,7 @@ class Item extends React.PureComponent {
           <video
             className='media-gallery__item-gifv-thumbnail'
             aria-label={attachment.get('description')}
+            title={attachment.get('description')}
             role='application'
             src={attachment.get('url')}
             onClick={this.handleClick}