chore: replace mattermost-redux with mattermost/client

Signed-off-by: Yukai Huang <yukaihuangtw@gmail.com>

Author: Yukaii

lib/auth/mattermost/index.js

@@ -3,7 +3,7 @@ require('babel-polyfill')
 require('isomorphic-fetch')
 const Router = require('express').Router
 const passport = require('passport')
-const MattermostClient = require('mattermost-redux/client/client4').default
+const { Client4: MattermostClient } = require('@mattermost/client')
 const OAuthStrategy = require('passport-oauth2').Strategy
 const config = require('../../config')
 const { setReturnToFromReferer, passportGeneralCallback } = require('../utils')