Add missing video descriptions (#8416)

* Pass through video description as necessary.

* Add title to video, matching aria-label.

app/javascript/mastodon/features/report/components/status_check_box.js

@@ -36,6 +36,7 @@ export default class StatusCheckBox extends React.PureComponent {
               <Component
                 preview={video.get('preview_url')}
                 src={video.get('url')}
+                alt={video.get('description')}
                 width={239}
                 height={110}
                 inline

app/javascript/mastodon/features/status/components/detailed_status.js

@@ -60,6 +60,7 @@ export default class DetailedStatus extends ImmutablePureComponent {
           <Video
             preview={video.get('preview_url')}
             src={video.get('url')}
+            alt={video.get('description')}
             width={300}
             height={150}
             inline

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}