Add missing video descriptions (#8416)

* Pass through video description as necessary.

* Add title to video, matching aria-label.

app/javascript/mastodon/features/video/index.js

@@ -315,6 +315,7 @@ export default class Video extends React.PureComponent {
           role='button'
           tabIndex='0'
           aria-label={alt}
+          title={alt}
           width={width}
           height={height}
           onClick={this.togglePlay}