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/activitypub/note_serializer.rb

@@ -89,12 +89,16 @@ class ActivityPub::NoteSerializer < ActiveModel::Serializer
   class MediaAttachmentSerializer < ActiveModel::Serializer
     include RoutingHelper
 
-    attributes :type, :media_type, :url
+    attributes :type, :media_type, :url, :name
 
     def type
       'Document'
     end
 
+    def name
+      object.description
+    end
+
     def media_type
       object.file_content_type
     end