Use Constants.MAX_CHANNELNAME_LENGTH where a fixed number like 22 or 64 is used for max length of channel names. (#7041)

webapp/utils/constants.jsx

@@ -1,6 +1,8 @@
 // Copyright (c) 2015-present Mattermost, Inc. All Rights Reserved.
 // See License.txt for license information.
 
+/* eslint-disable no-magic-numbers */
+
 import keyMirror from 'key-mirror';
 
 import audioIcon from 'images/icons/audio.png';