Add ability to specify alternative text for media attachments (#5123)

* Fix #117 - Add ability to specify alternative text for media attachments

- POST /api/v1/media accepts `description` straight away
- PUT /api/v1/media/:id to update `description` (only for unattached ones)
- Serialized as `name` of Document object in ActivityPub
- Uploads form adjusted for better performance and description input

* Add tests

* Change undo button blend mode to difference

app/javascript/mastodon/components/extended_video_player.js

@@ -5,6 +5,7 @@ export default class ExtendedVideoPlayer extends React.PureComponent {
 
   static propTypes = {
     src: PropTypes.string.isRequired,
+    alt: PropTypes.string,
     width: PropTypes.number,
     height: PropTypes.number,
     time: PropTypes.number,
@@ -31,15 +32,20 @@ export default class ExtendedVideoPlayer extends React.PureComponent {
   }
 
   render () {
+    const { src, muted, controls, alt } = this.props;
+
     return (
       <div className='extended-video-player'>
         <video
           ref={this.setRef}
-          src={this.props.src}
+          src={src}
           autoPlay
-          muted={this.props.muted}
-          controls={this.props.controls}
-          loop={!this.props.controls}
+          role='button'
+          tabIndex='0'
+          aria-label={alt}
+          muted={muted}
+          controls={controls}
+          loop={!controls}
         />
       </div>
     );