tag change notices: use cache instead of using redis directly.

app/logical/tag_change_notice_service.rb

@@ -1,18 +1,13 @@
 module TagChangeNoticeService
   extend self
 
-  def redis_client
-    ::Redis.new(url: Danbooru.config.redis_url)
-  end
-
   def get_forum_topic_id(tag)
-    redis_client.get("tcn:#{tag}")
+    Cache.get("tcn:#{tag}")
   end
 
   def update_cache(affected_tags, forum_topic_id)
-    rc = redis_client
     affected_tags.each do |tag|
-      rc.setex("tcn:#{tag}", 1.week, forum_topic_id)
+      Cache.put("tcn:#{tag}", forum_topic_id, 1.week)
     end
   end
 end