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/features/ui/components/video_modal.js

@@ -23,6 +23,7 @@ export default class VideoModal extends ImmutablePureComponent {
             src={media.get('url')}
             startTime={time}
             onCloseVideo={onClose}
+            description={media.get('description')}
           />
         </div>
       </div>