…#604

Closes #604

Checklist

• I have read the OpenWISP Contributing Guidelines.
• I have manually tested the changes proposed in this pull request.
• I have written new test cases for new code and/or updated existing tests for changes to existing code.
• I have updated the documentation.

Reference to Existing Issue

Closes #604 .

Description of Changes

• Added captive-portal-handler.js to handle authentication failure detection and error messaging.
• When a captive portal authentication fails, it redirects back to the login page.

Also:

• By changing to:

import LoginComponent from "../login";

• We're getting the default export (the connected component) and there's no ambiguity about which version we're using as I was getting ESLint error