タグ “廃止” 追加 (#378) (#379)

Reviewed-on: http://git.miteruzo.com/miteruzo/btrc-hub/pulls/379
Co-authored-by: miteruzo <miteruzo@naver.com>
Co-committed-by: miteruzo <miteruzo@naver.com>

backend/app/models/wiki_page.rb

@@ -22,6 +22,7 @@ class WikiPage < ApplicationRecord
   validates :body, presence: true
 
   def title = tag_name.name
+  def deprecated_at = tag_name.tag&.deprecated_at
 
   def title= val
     (self.tag_name ||= build_tag_name).name = val