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

config/routes.rb

@@ -193,7 +193,7 @@ Rails.application.routes.draw do
       get '/search', to: 'search#index', as: :search
 
       resources :follows,    only: [:create]
-      resources :media,      only: [:create]
+      resources :media,      only: [:create, :update]
       resources :apps,       only: [:create]
       resources :blocks,     only: [:index]
       resources :mutes,      only: [:index]