add i18n and a todo comment

resource/locales/en_US/translation.json

@@ -109,6 +109,7 @@
   "Markdown Settings": "Markdown Settings",
   "Customize": "Customize",
   "Notification Settings": "Notification Settings",
+  "Legacy_External_Notification": "Legacy External Notification",
   "User_Management": "User Management",
   "external_account_management": "External Account Management",
   "UserGroup Management": "UserGroup Management",

src/server/views/admin/legacy-external-notification.html

@@ -7,7 +7,6 @@
 {% endblock %}
 
 {% block content_main %}
-hoge
-<!-- <div id="admin-markdown-setting"></div> -->
+<!-- TODO: move contents from notification settings by GW-5467  -->
 {% endblock content_main %}