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/media_gallery.js

@@ -136,7 +136,7 @@ class Item extends React.PureComponent {
           onClick={this.handleClick}
           target='_blank'
         >
-          <img src={previewUrl} srcSet={srcSet} sizes={sizes} alt='' />
+          <img src={previewUrl} srcSet={srcSet} sizes={sizes} alt={attachment.get('description')} />
         </a>
       );
     } else if (attachment.get('type') === 'gifv') {
@@ -146,6 +146,7 @@ class Item extends React.PureComponent {
         <div className={classNames('media-gallery__gifv', { autoplay: autoPlay })}>
           <video
             className='media-gallery__item-gifv-thumbnail'
+            aria-label={attachment.get('description')}
             role='application'
             src={attachment.get('url')}
             onClick={this.handleClick}