Clear management auth storage when redirecting to login

This branch redirects the client to the login page when the cookie
expires. To complete the logout process we should also clear any auth
data stored in local storage: local storage has no built-in expiration
mechanism.

To test this locally you can use `make run-broker`, set the session
timeout to one minute for quick testing:

    application:set_env(rabbitmq_management, login_session_timeout, 1)

go to the management page (`http://localhost:15672/#/`), login with
default credentials and wait a minute. After this change the local
storage only contains info like `rabbitmq.vhost` and `rabbitmq.version`.

deps/rabbitmq_management/priv/www/js/main.js

@@ -1316,7 +1316,8 @@ function update_status(status) {
 
 function with_req(method, path, body, fun) {
     if(!has_auth_credentials()) {
-        // navigate to the login form
+        // Clear any lingering auth settings in local storage and navigate to the login form.
+        clear_auth();
         location.reload();
         return;
     }