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

db/migrate/20170927215609_add_description_to_media_attachments.rb

@@ -0,0 +1,5 @@
+class AddDescriptionToMediaAttachments < ActiveRecord::Migration[5.1]
+  def change
+    add_column :media_attachments, :description, :text
+  end
+end