Fix #5227: Error on note previews due to URL length limit

Allow either GET or POST for /note_previews.

config/routes.rb

@@ -172,7 +172,7 @@ Rails.application.routes.draw do
     end
   end
   resources :note_versions, :only => [:index, :show]
-  resource :note_previews, :only => [:show]
+  resource :note_previews, only: [:create, :show]
   resource :password_reset, only: [:create, :show]
   resources :pixiv_ugoira_frame_data, only: [:index]
   resources :pools do