Fix media markup (#21420)

This brings the markup of the MediaItem component on par with the Item component from media_gallery.

Co-authored-by: Effy Elden <effy@effy.space>

app/javascript/mastodon/features/account_gallery/components/media_item.js

@@ -104,6 +104,7 @@ export default class MediaItem extends ImmutablePureComponent {
           <video
             className='media-gallery__item-gifv-thumbnail'
             aria-label={attachment.get('description')}
+            title={attachment.get('description')}
             role='application'
             src={attachment.get('url')}
             onMouseEnter={this.handleMouseEnter}