use app:title for username to notify

lib/util/slack.js

@@ -206,7 +206,7 @@ module.exports = function(crowi) {
 
     var message = {
       channel: '#' + channel,
-      username: 'Crowi',
+      username: config.crowi['app:title'],
       text: this.getSlackMessageText(page.path, user, updateType),
       attachments: [attachment],
     };