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/serializers/rest/media_attachment_serializer.rb

@@ -4,7 +4,8 @@ class REST::MediaAttachmentSerializer < ActiveModel::Serializer
   include RoutingHelper
 
   attributes :id, :type, :url, :preview_url,
-             :remote_url, :text_url, :meta
+             :remote_url, :text_url, :meta,
+             :description
 
   def id
     object.id.to_s