Alter Japanese translations for CW and NSFW (#2305)

The previous translation for NSFW, "不適切" means "impertinent". This change
suggests "閲覧注意", which means the content requires precaution, instead.

However, "閲覧注意" was also a translation for CW. CW is now translated as
"警告", which simply means "warnings".

config/locales/ja.yml

@@ -288,7 +288,7 @@ ja:
   stream_entries:
     click_to_show: クリックして表示
     reblogged: ブーストされました
-    sensitive_content: 不適切なコンテンツの可能性があります
+    sensitive_content: 閲覧注意
   time:
     formats:
       default: "%Y年%m月%d日 %H:%M"