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/util/async-components.js

@@ -90,10 +90,6 @@ export function MediaGallery () {
   return import(/* webpackChunkName: "status/media_gallery" */'../../../components/media_gallery');
 }
 
-export function VideoPlayer () {
-  return import(/* webpackChunkName: "status/video_player" */'../../../components/video_player');
-}
-
 export function Video () {
   return import(/* webpackChunkName: "features/video" */'../../video');
 }