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/controllers/api/v1/media_controller.rb

@@ -10,7 +10,7 @@ class Api::V1::MediaController < Api::BaseController
   respond_to :json
 
   def create
-    @media = current_account.media_attachments.create!(file: media_params[:file])
+    @media = current_account.media_attachments.create!(media_params)
     render json: @media, serializer: REST::MediaAttachmentSerializer
   rescue Paperclip::Errors::NotIdentifiedByImageMagickError
     render json: file_type_error, status: 422
@@ -18,10 +18,16 @@ class Api::V1::MediaController < Api::BaseController
     render json: processing_error, status: 500
   end
 
+  def update
+    @media = current_account.media_attachments.where(status_id: nil).find(params[:id])
+    @media.update!(media_params)
+    render json: @media, serializer: REST::MediaAttachmentSerializer
+  end
+
   private
 
   def media_params
-    params.permit(:file)
+    params.permit(:file, :description)
   end
 
   def file_type_error