add: system notification management

app/models/system_notification.rb

@@ -0,0 +1,17 @@
+# == Schema Information
+#
+# Table name: system_notifications
+#
+#  id          :integer          not null, primary key
+#  subject     :string(255)
+#  sub_subject :string(255)
+#  content     :string(255)
+#  created_at  :datetime         not null
+#  updated_at  :datetime         not null
+#
+
+class SystemNotification < ApplicationRecord
+
+  has_many :system_notification_histories
+  has_many :users, through: :system_notification_histories
+end