add: message template content

app/models/message_template/pull_request_changed.rb

@@ -2,12 +2,13 @@
 #
 # Table name: message_templates
 #
-#  id         :integer          not null, primary key
-#  type       :string(255)
-#  sys_notice :text(65535)
-#  email      :text(65535)
-#  created_at :datetime         not null
-#  updated_at :datetime         not null
+#  id               :integer          not null, primary key
+#  type             :string(255)
+#  sys_notice       :text(65535)
+#  email            :text(65535)
+#  created_at       :datetime         not null
+#  updated_at       :datetime         not null
+#  notification_url :string(255)
 #
 
 # 我创建或负责的合并请求状态变更