Treat the literal team token string "undefined" as undefined

Some users appear to have gotten team tokens into their local storage. This fix will treat the literal string "undefined" as undefined.

src/components/structures/MatrixChat.js

@@ -217,6 +217,12 @@ module.exports = React.createClass({
             window.localStorage.getItem('mx_team_token') ||
             window.sessionStorage.getItem('mx_team_token');
 
+        // Some users have ended up with "undefined" as their local storage team token,
+        // treat that as undefined.
+        if (this._teamToken === "undefined") {
+            this._teamToken = undefined;
+        }
+
         if (this._teamToken) {
             console.info(`Team token set to ${this._teamToken}`);
         }